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

Log Message:
-----------
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.

Modified Files:
--------------
pgsql/src/backend/utils/adt:
varlena.c (r1.156 -> r1.157)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/varlena.c.diff?r1=1.156&r2=1.157)