pgsql: Fix for rearranging encoding id ISO-8859-5 to ISO-8859-8.

Log Message:
-----------
Fix for rearranging encoding id ISO-8859-5 to ISO-8859-8.
Also make the code more robust by searching for target encoding
in the internal charset map.

Problem reported by Sagi Bashari on 2005/12/21.
See "[BUGS] BUG #2120: Crash when doing UTF8<->ISO_8859_8 encoding conversion"
on pgsql-bugs list for more details.

Back patch for 8.1_STABLE.

Tags:
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REL8_1_STABLE

Modified Files:
--------------
    pgsql/src/backend/utils/mb/conversion_procs/utf8_and_iso8859:
        utf8_and_iso8859.c (r1.15.2.1 -> r1.15.2.2)

(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/mb/conversion_procs/utf8_and_iso8859/utf8_and_iso8859.c.diff?r1=1.15.2.1&r2=1.15.2.2)