appletalk: Fix OOPS in atalk_release().

Commit 60d9f461 ("appletalk: remove
the BKL") added a dereference of "sk" before checking for NULL in
atalk_release().

Guard the code block completely, rather than partially, with the
NULL check.
Reported-by: NDave Jones <davej@redhat.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/appletalk/ddp.c

@@ -1051,16 +1051,17 @@ static int atalk_release(struct socket *sock)
 {
 	struct sock *sk = sock->sk;
 
-	sock_hold(sk);
-	lock_sock(sk);
 	if (sk) {
+		sock_hold(sk);
+		lock_sock(sk);
+
 		sock_orphan(sk);
 		sock->sk = NULL;
 		atalk_destroy_socket(sk);
-	}
-	release_sock(sk);
-	sock_put(sk);
 
+		release_sock(sk);
+		sock_put(sk);
+	}
 	return 0;
 }