Plane: Quadplane: check for valid RC before passing throttle to outputs

[IamPete1]

Extract from #25416

This ensures that pilot throttle is valid before outputting to motors. These two special cases are a outside of our typical code paths so its a good idea to double check.

In both cases currently it is possible that the throttle will be hard coded to 50% here:

ardupilot/ArduPlane/radio.cpp

Lines 261 to 274 in ee59247

	case Mode::Number::QSTABILIZE:
	case Mode::Number::QHOVER:
	case Mode::Number::QLOITER:
	case Mode::Number::QLAND: // throttle is ignored, but reset anyways
	case Mode::Number::QRTL: // throttle is ignored, but reset anyways
	case Mode::Number::QACRO:
	#if QAUTOTUNE_ENABLED
	case Mode::Number::QAUTOTUNE:
	#endif
	if (quadplane.available() && quadplane.motors->get_desired_spool_state() > AP_Motors::DesiredSpoolState::GROUND_IDLE) {
	// set half throttle to avoid descending at maximum rate, still has a slight descent due to throttle deadzone
	channel_throttle->set_control_in(channel_throttle->get_range() / 2);
	break;
	}

I suspect its fine and we keep the spool state at shut down but this ensures we never output the wrong thing.

[IamPete1]

I found the problem affected more areas than expected so went with a more general fix, #25633