pgsql: Replace argument-checking Asserts with regular test-and-elog

Log Message:
-----------
Replace argument-checking Asserts with regular test-and-elog checks in all
encoding conversion functions.  These are not can't-happen cases because
it's possible to create a conversion with the wrong conversion function
for the specified encoding pair.  That would lead to an Assert crash in
an Assert-enabled build, or incorrect conversion otherwise, neither of
which is desirable.  This would be a DOS issue if production databases
were customarily built with asserts enabled, but fortunately that's not so.
Per an observation by Heikki.

Back-patch to all supported branches.

Modified Files:
--------------
    pgsql/src/backend/utils/mb/conversion_procs/ascii_and_mic:
        ascii_and_mic.c (r1.15 -> r1.16)

(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/mb/conversion_procs/ascii_and_mic/ascii_and_mic.c?r1=1.15&r2=1.16)
    pgsql/src/backend/utils/mb/conversion_procs/cyrillic_and_mic:
        cyrillic_and_mic.c (r1.18 -> r1.19)

(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/mb/conversion_procs/cyrillic_and_mic/cyrillic_and_mic.c?r1=1.18&r2=1.19)
    pgsql/src/backend/utils/mb/conversion_procs/euc_cn_and_mic:
        euc_cn_and_mic.c (r1.18 -> r1.19)

(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/mb/conversion_procs/euc_cn_and_mic/euc_cn_and_mic.c?r1=1.18&r2=1.19)
    pgsql/src/backend/utils/mb/conversion_procs/euc_jis_2004_and_shift_jis_2004:
        euc_jis_2004_and_shift_jis_2004.c (r1.5 -> r1.6)

(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/mb/conversion_procs/euc_jis_2004_and_shift_jis_2004/euc_jis_2004_and_shift_jis_2004.c?r1=1.5&r2=1.6)
    pgsql/src/backend/utils/mb/conversion_procs/euc_jp_and_sjis:
        euc_jp_and_sjis.c (r1.20 -> r1.21)

(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/mb/conversion_procs/euc_jp_and_sjis/euc_jp_and_sjis.c?r1=1.20&r2=1.21)
    pgsql/src/backend/utils/mb/conversion_procs/euc_kr_and_mic:
        euc_kr_and_mic.c (r1.18 -> r1.19)

(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/mb/conversion_procs/euc_kr_and_mic/euc_kr_and_mic.c?r1=1.18&r2=1.19)
    pgsql/src/backend/utils/mb/conversion_procs/euc_tw_and_big5:
        euc_tw_and_big5.c (r1.19 -> r1.20)

(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/mb/conversion_procs/euc_tw_and_big5/euc_tw_and_big5.c?r1=1.19&r2=1.20)
    pgsql/src/backend/utils/mb/conversion_procs/latin2_and_win1250:
        latin2_and_win1250.c (r1.16 -> r1.17)

(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/mb/conversion_procs/latin2_and_win1250/latin2_and_win1250.c?r1=1.16&r2=1.17)
    pgsql/src/backend/utils/mb/conversion_procs/latin_and_mic:
        latin_and_mic.c (r1.16 -> r1.17)

(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/mb/conversion_procs/latin_and_mic/latin_and_mic.c?r1=1.16&r2=1.17)
    pgsql/src/backend/utils/mb/conversion_procs/utf8_and_ascii:
        utf8_and_ascii.c (r1.17 -> r1.18)

(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/mb/conversion_procs/utf8_and_ascii/utf8_and_ascii.c?r1=1.17&r2=1.18)
    pgsql/src/backend/utils/mb/conversion_procs/utf8_and_big5:
        utf8_and_big5.c (r1.19 -> r1.20)

(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/mb/conversion_procs/utf8_and_big5/utf8_and_big5.c?r1=1.19&r2=1.20)
    pgsql/src/backend/utils/mb/conversion_procs/utf8_and_cyrillic:
        utf8_and_cyrillic.c (r1.22 -> r1.23)

(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/mb/conversion_procs/utf8_and_cyrillic/utf8_and_cyrillic.c?r1=1.22&r2=1.23)
    pgsql/src/backend/utils/mb/conversion_procs/utf8_and_euc_cn:
        utf8_and_euc_cn.c (r1.20 -> r1.21)

(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/mb/conversion_procs/utf8_and_euc_cn/utf8_and_euc_cn.c?r1=1.20&r2=1.21)
    pgsql/src/backend/utils/mb/conversion_procs/utf8_and_euc_jis_2004:
        utf8_and_euc_jis_2004.c (r1.4 -> r1.5)

(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/mb/conversion_procs/utf8_and_euc_jis_2004/utf8_and_euc_jis_2004.c?r1=1.4&r2=1.5)
    pgsql/src/backend/utils/mb/conversion_procs/utf8_and_euc_jp:
        utf8_and_euc_jp.c (r1.20 -> r1.21)

(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/mb/conversion_procs/utf8_and_euc_jp/utf8_and_euc_jp.c?r1=1.20&r2=1.21)
    pgsql/src/backend/utils/mb/conversion_procs/utf8_and_euc_kr:
        utf8_and_euc_kr.c (r1.20 -> r1.21)

(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/mb/conversion_procs/utf8_and_euc_kr/utf8_and_euc_kr.c?r1=1.20&r2=1.21)
    pgsql/src/backend/utils/mb/conversion_procs/utf8_and_euc_tw:
        utf8_and_euc_tw.c (r1.20 -> r1.21)

(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/mb/conversion_procs/utf8_and_euc_tw/utf8_and_euc_tw.c?r1=1.20&r2=1.21)
    pgsql/src/backend/utils/mb/conversion_procs/utf8_and_gb18030:
        utf8_and_gb18030.c (r1.21 -> r1.22)

(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/mb/conversion_procs/utf8_and_gb18030/utf8_and_gb18030.c?r1=1.21&r2=1.22)
    pgsql/src/backend/utils/mb/conversion_procs/utf8_and_gbk:
        utf8_and_gbk.c (r1.18 -> r1.19)

(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/mb/conversion_procs/utf8_and_gbk/utf8_and_gbk.c?r1=1.18&r2=1.19)
    pgsql/src/backend/utils/mb/conversion_procs/utf8_and_iso8859:
        utf8_and_iso8859.c (r1.29 -> r1.30)

(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/mb/conversion_procs/utf8_and_iso8859/utf8_and_iso8859.c?r1=1.29&r2=1.30)
    pgsql/src/backend/utils/mb/conversion_procs/utf8_and_iso8859_1:
        utf8_and_iso8859_1.c (r1.21 -> r1.22)

(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/mb/conversion_procs/utf8_and_iso8859_1/utf8_and_iso8859_1.c?r1=1.21&r2=1.22)
    pgsql/src/backend/utils/mb/conversion_procs/utf8_and_johab:
        utf8_and_johab.c (r1.20 -> r1.21)

(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/mb/conversion_procs/utf8_and_johab/utf8_and_johab.c?r1=1.20&r2=1.21)
    pgsql/src/backend/utils/mb/conversion_procs/utf8_and_shift_jis_2004:
        utf8_and_shift_jis_2004.c (r1.4 -> r1.5)

(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/mb/conversion_procs/utf8_and_shift_jis_2004/utf8_and_shift_jis_2004.c?r1=1.4&r2=1.5)
    pgsql/src/backend/utils/mb/conversion_procs/utf8_and_sjis:
        utf8_and_sjis.c (r1.18 -> r1.19)

(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/mb/conversion_procs/utf8_and_sjis/utf8_and_sjis.c?r1=1.18&r2=1.19)
    pgsql/src/backend/utils/mb/conversion_procs/utf8_and_uhc:
        utf8_and_uhc.c (r1.18 -> r1.19)

(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/mb/conversion_procs/utf8_and_uhc/utf8_and_uhc.c?r1=1.18&r2=1.19)
    pgsql/src/backend/utils/mb/conversion_procs/utf8_and_win:
        utf8_and_win.c (r1.13 -> r1.14)

(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/mb/conversion_procs/utf8_and_win/utf8_and_win.c?r1=1.13&r2=1.14)
    pgsql/src/backend/utils/mb:
        wchar.c (r1.68 -> r1.69)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/mb/wchar.c?r1=1.68&r2=1.69)
    pgsql/src/include/mb:
        pg_wchar.h (r1.82 -> r1.83)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/mb/pg_wchar.h?r1=1.82&r2=1.83)