rtl: fixed computation of return altitude when mission landing is available

Signed-off-by: RomanBapst <bapstroman@gmail.com>

src/modules/navigator/rtl.cpp

@@ -218,7 +218,8 @@ void RTL::on_activation()
 		_rtl_alt = calculate_return_alt_from_cone_half_angle((float)_param_rtl_cone_half_angle_deg.get());
 
 	} else {
-		_rtl_alt = math::max(global_position.alt, _destination.alt + _param_rtl_return_alt.get());
+		_rtl_alt = max(global_position.alt, max(_destination.alt,
+							_navigator->get_home_position()->alt + _param_rtl_return_alt.get()));
 	}
 
 	if (_navigator->get_land_detected()->landed) {