MotorSingle: check servo_out above min_throttle

We need to recalc radio_out or the motors could fall below min throttle

libraries/AP_Motors/AP_MotorsSingle.cpp

@@ -172,6 +172,8 @@ void AP_MotorsSingle::output_armed()
         // check if throttle is at or below limit
         if (_rc_throttle.servo_out <= _min_throttle) {
             limit.throttle_lower = true;
+            _rc_throttle.servo_out = _min_throttle;
+            _rc_throttle.calc_pwm();    // recalculate radio.out
         }
 
 		//motor