mavlink: mission minor whitespace fixes and initialization
mavlink: mission minor whitespace fixes and initialization

 - it's safer to zero any struct being send over mavlink to prevent
extension field surpries later

krbeverx/Firmware,acfloria/Firmware,krbeverx/Firmware,acfloria/Firmware,PX4/Firmware,PX4/Firmware,acfloria/Firmware,krbeverx/Firmware,acfloria/Firmware,PX4/Firmware,acfloria/Firmware,acfloria/Firmware,PX4/Firmware,PX4/Firmware,krbeverx/Firmware,acfloria/Firmware,krbeverx/Firmware,PX4/Firmware,krbeverx/Firmware,krbeverx/Firmware,PX4/Firmware