Fix stale signal problem

src/comm/MAVLinkProtocol.cc

@@ -223,6 +223,13 @@ void MAVLinkProtocol::_linkStatusChanged(LinkInterface* link, bool connected)
  **/
 void MAVLinkProtocol::receiveBytes(LinkInterface* link, QByteArray b)
 {
+    // Since receiveBytes signals cross threads we can end up with signals in the queue
+    // that come through after the link is disconnected. For these we just drop the data
+    // since the link is closed.
+    if (!LinkManager::instance()->containsLink(link)) {
+        return;
+    }
+    
 //    receiveMutex.lock();
     mavlink_message_t message;
     mavlink_status_t status;