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.

src/backend/regex/regcomp.c

@@ -278,6 +278,9 @@ static struct fns functions = {
 
 /*
  * pg_regcomp - compile regular expression
+ *
+ * Note: on failure, no resources remain allocated, so pg_regfree()
+ * need not be applied to re.
  */
 int
 pg_regcomp(regex_t *re,
@@ -1870,15 +1873,18 @@ rfree(regex_t *re)
 	g = (struct guts *) re->re_guts;
 	re->re_guts = NULL;
 	re->re_fns = NULL;
-	g->magic = 0;
-	freecm(&g->cmap);
-	if (g->tree != NULL)
-		freesubre((struct vars *) NULL, g->tree);
-	if (g->lacons != NULL)
-		freelacons(g->lacons, g->nlacons);
-	if (!NULLCNFA(g->search))
-		freecnfa(&g->search);
-	FREE(g);
+	if (g != NULL)
+	{
+		g->magic = 0;
+		freecm(&g->cmap);
+		if (g->tree != NULL)
+			freesubre((struct vars *) NULL, g->tree);
+		if (g->lacons != NULL)
+			freelacons(g->lacons, g->nlacons);
+		if (!NULLCNFA(g->search))
+			freecnfa(&g->search);
+		FREE(g);
+	}
 }
 
 #ifdef REG_DEBUG

src/backend/utils/adt/regexp.c

@@ -187,9 +187,8 @@ RE_compile_and_cache(text *text_re, int cflags, Oid collation)
 
 	if (regcomp_result != REG_OKAY)
 	{
-		/* re didn't compile */
+		/* re didn't compile (no need for pg_regfree, if so) */
 		pg_regerror(regcomp_result, &re_temp.cre_re, errMsg, sizeof(errMsg));
-		/* XXX should we pg_regfree here? */
 		ereport(ERROR,
 				(errcode(ERRCODE_INVALID_REGULAR_EXPRESSION),
 				 errmsg("invalid regular expression: %s", errMsg)));