GCS_MAVLink: mark as not receiving when too many items in MISSION_COUNT.count

we left things in the "receiving" state, which would eventually lead to a timeout, rather than just failing instanenously with the correct code.

libraries/GCS_MAVLink/MissionItemProtocol.cpp

@@ -73,6 +73,8 @@ void MissionItemProtocol::handle_mission_count(
         // the upload count may have changed; free resources and
         // allocate them again:
         free_upload_resources();
+        receiving = false;
+        link = nullptr;
     }
 
     if (packet.count > max_items()) {