tipc: fix a potential oops

Commit 6c9808ce ("tipc: remove port_lock") accidentally involves
a potential bug: when tipc socket instance(tsk) is not got with given
reference number in tipc_sk_get(), tsk is set to NULL. Subsequently
we jump to exit label where to decrease socket reference counter
pointed by tsk pointer in tipc_sk_put(). However, As now tsk is NULL,
oops may happen because of touching a NULL pointer.
Signed-off-by: Ying Xue <ying.xue@windriver.com>
Acked-by: Erik Hugne <erik.hugne@ericsson.com>
Acked-by: Jon Maloy <jon.maloy@ericsson.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/tipc/socket.c

@@ -2118,9 +2118,9 @@ static void tipc_sk_timeout(unsigned long ref)
 
 	tsk = tipc_sk_get(ref);
 	if (!tsk)
-		goto exit;
-	sk = &tsk->sk;
+		return;
 
+	sk = &tsk->sk;
 	bh_lock_sock(sk);
 	if (!tsk->connected) {
 		bh_unlock_sock(sk);