CRED: Allow put_cred() to cope with a NULL groups list

put_cred() will oops if given a NULL groups list, but that is now possible with
the existence of cred_alloc_blank(), as used in keyctl_session_to_parent().

Added in commit:

	commit ee18d64c1f632043a02e6f5ba5e045bb26a5465f
	Author: David Howells <dhowells@redhat.com>
	Date:   Wed Sep 2 09:14:21 2009 +0100
	KEYS: Add a keyctl to install a process's session keyring on its parent [try #6]

Reported-by: Marc Dionne <marc.c.dionne@gmail.com>
Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: James Morris <jmorris@namei.org>

kernel/cred.c

@@ -147,7 +147,8 @@ static void put_cred_rcu(struct rcu_head *rcu)
 	key_put(cred->thread_keyring);
 	key_put(cred->request_key_auth);
 	release_tgcred(cred);
-	put_group_info(cred->group_info);
+	if (cred->group_info)
+		put_group_info(cred->group_info);
 	free_uid(cred->user);
 	kmem_cache_free(cred_jar, cred);
 }