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.