mixer_module: more robust logic to set 'stop_motors' flag
mixer_module: more robust logic to set 'stop_motors' flag

Checking the first output_limited for a disarmed value is fragile.
For example a disarmed value might be within the range of min/max output
values and could then be triggered while armed.

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