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.

Author: peterbarker

libraries/GCS_MAVLink/MissionItemProtocol.cpp

@@ -79,6 +79,8 @@ void MissionItemProtocol::handle_mission_count(
         // FIXME: different items take up different storage space!
         send_mission_ack(_link, msg, MAV_MISSION_NO_SPACE);
         GCS_SEND_TEXT(MAV_SEVERITY_WARNING, "Only %u items are supported", (unsigned)max_items());
+        receiving = false;
+        link = nullptr;
         return;
     }