pgsql: Prevent corner-case core dump in rfree().

Prevent corner-case core dump in rfree().

rfree() failed to cope with the case that pg_regcomp() had initialized the
regex_t struct but then failed to allocate any memory for re->re_guts (ie,
the first malloc call in pg_regcomp() failed). It would try to touch the
guts struct anyway, and thus dump core. This is a sufficiently narrow
corner case that it's not surprising it's never been seen in the field;
but still a bug is a bug, so patch all active branches.

Noted while investigating whether we need to call pg_regfree after a
failure return from pg_regcomp. Other than this bug, it turns out we
don't, so adjust comments appropriately.

Branch
------
REL8_3_STABLE

Details
-------
http://git.postgresql.org/pg/commitdiff/b277a938ea79e96926e4544eed2e800da1228176

Modified Files
--------------
src/backend/regex/regcomp.c | 24 +++++++++++++++---------
src/backend/utils/adt/regexp.c | 3 +--
2 files changed, 16 insertions(+), 11 deletions(-)