fix joystick: avoid accessing invalid pointer after vehicle is destroyed
fix joystick: avoid accessing invalid pointer after vehicle is destroyed

Joystick::_activeVehicle was still pointing to a vehicle after it got
destroyed (e.g. on disconnect) and the thread continued to access it.

Ideally Joystick::_activeVehicle would be changed to a weak pointer.

Hunter522/qgroundcontrol,Hunter522/qgroundcontrol,Hunter522/qgroundcontrol,Hunter522/qgroundcontrol,Hunter522/qgroundcontrol,Hunter522/qgroundcontrol