pgsql: Make replace(), split_part(), and string_to_array() behave

Log Message:
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Make replace(), split_part(), and string_to_array() behave somewhat sanely
when handed an invalidly-encoded pattern.  The previous coding could get
into an infinite loop if pg_mb2wchar_with_len() returned a zero-length
string after we'd tested for nonempty pattern; which is exactly what it
will do if the string consists only of an incomplete multibyte character.
This led to either an out-of-memory error or a backend crash depending
on platform.  Per report from Wiktor Wodecki.

Tags:
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REL8_2_STABLE

Modified Files:
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    pgsql/src/backend/utils/adt:
        varlena.c (r1.153 -> r1.153.2.1)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/varlena.c.diff?r1=1.153&r2=1.153.2.1)