Fix memory leak during regular expression execution.

For a regex containing backrefs, pg_regexec() might fail to free all the
sub-DFAs that were created during execution, resulting in a permanent
(session lifespan) memory leak.  Problem was introduced by me in commit
58735947.  Per report from Sandro Santilli;
diagnosis by Greg Stark.

src/backend/regex/regexec.c

@@ -259,6 +259,7 @@ pg_regexec(regex_t *re,
 	/* clean up */
 	if (v->pmatch != pmatch && v->pmatch != mat)
 		FREE(v->pmatch);
+	n = (size_t) v->g->ntree;
 	for (i = 0; i < n; i++)
 	{
 		if (v->subdfas[i] != NULL)