pgsql: Avoid memory leakage across successive calls of regexp_matches()

Log Message:
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Avoid memory leakage across successive calls of regexp_matches() or
regexp_split_to_table() within a single query.  This is only a partial
solution, as it turns out that with enough matches per string these
functions can also tickle a repalloc() misbehavior.  But fixing that
is a topic for a separate patch.

Modified Files:
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    pgsql/src/backend/utils/adt:
        regexp.c (r1.72 -> r1.73)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/regexp.c?r1=1.72&r2=1.73)