mc_pos_control: fix takeoff bug

This resolves a bug where a takeoff would go sideways instead of
straight up. What happened was that the position setpoint got shifted
around even though there was actually no real setpoint set but only a
setpoint of type IDLE. This then lead to a position setpoint far away
from the takeoff point and therefore scary takeoffs.

This fix prevents the part of the position controller which
moves/integrates the position setpoint from running in the case of an
idle setpoint.

This bug could be reproduced by switching the vehicle to mission mode
without a valid mission, then switch to hold mode, and then send the arm
and takeoff command.

src/modules/mc_pos_control/mc_pos_control_main.cpp

@@ -1144,7 +1144,9 @@ void MulticopterPositionControl::control_auto(float dt)
 		}
 	}
 
-	if (current_setpoint_valid) {
+	if (current_setpoint_valid &&
+	    (_pos_sp_triplet.current.type != position_setpoint_s::SETPOINT_TYPE_IDLE)) {
+
 		/* scaled space: 1 == position error resulting max allowed speed */
 
 		math::Vector<3> cruising_speed = _params.vel_cruise;