fix mavlink: ensure vehicle_command topic exists
fix mavlink: ensure vehicle_command topic exists

If the first publisher publishes multiple commands right after each other
(e.g. on 'commander takeoff'), mavlink would miss the first and print an
error like 'vehicle_command lost, generation 0 -> 2'.

This is due to a recent uORB behavior change.

acfloria/Firmware,acfloria/Firmware,acfloria/Firmware,krbeverx/Firmware,krbeverx/Firmware,acfloria/Firmware,acfloria/Firmware,krbeverx/Firmware,krbeverx/Firmware,krbeverx/Firmware,acfloria/Firmware,acfloria/Firmware,krbeverx/Firmware,krbeverx/Firmware