Avoid low-probability crash on out-of-memory.

check_restrict_nonsystem_relation_kind() correctly uses guc_malloc()
in v16 and later.  But in older branches it must use malloc()
directly, and it forgot to check for failure return.
Faulty backpatching of 66e94448a.

Karina Litskevich

Discussion: https://postgr.es/m/CACiT8iZ=atkguKVbpN4HmJFMb4+T9yEowF5JuPZG8W+kkZ9L6w@mail.gmail.com

src/backend/tcop/postgres.c

@@ -3649,6 +3649,11 @@ check_restrict_nonsystem_relation_kind(char **newval, void **extra, GucSource so
 
 	/* Save the flags in *extra, for use by the assign function */
 	*extra = malloc(sizeof(int));
+	if (*extra == NULL)
+		ereport(ERROR,
+				(errcode(ERRCODE_OUT_OF_MEMORY),
+				 errmsg("out of memory")));
+
 	*((int *) *extra) = flags;
 
 	return true;