MAVLink: Disconnect link if it does not contain protocol data
MAVLink: Disconnect link if it does not contain protocol data

This is important to avoid having QGC hijack links that are active (we read 2000 bytes!) but are definitely not MAVLink. This can have nasty side-effects, e.g. if we would talk to an LTE modem in a laptop or alike.

Hunter522/qgroundcontrol,ethz-asl/qgc_asl,Hunter522/qgroundcontrol,ethz-asl/qgc_asl,ethz-asl/qgc_asl,Hunter522/qgroundcontrol,Hunter522/qgroundcontrol,Hunter522/qgroundcontrol,ethz-asl/qgc_asl,Hunter522/qgroundcontrol,ethz-asl/qgc_asl