mc_pos_control: improve failsafe handling

See issue #12307

Since commander should still handle all failsafes we should only run
into this case as last resort to not crash.
If all failsafe actions are disabled but data is missing
e.g. RC loss action disabled but flying in manual and no RC
this can be tested.

src/modules/mc_pos_control/mc_pos_control_main.cpp

@@ -963,21 +963,34 @@ MulticopterPositionControl::failsafe(vehicle_local_position_setpoint_s &setpoint
 	} else {
 		reset_setpoint_to_nan(setpoint);
 
-		if (PX4_ISFINITE(_states.velocity(2))) {
-			// We have a valid velocity in D-direction.
-			// descend downwards with landspeed.
-			setpoint.vz = _param_mpc_land_speed.get();
-			setpoint.thrust[0] = setpoint.thrust[1] = 0.0f;
+		if (PX4_ISFINITE(_states.velocity(0)) && PX4_ISFINITE(_states.velocity(1))) {
+			// don't move along xy
+			setpoint.vx = setpoint.vy = 0.0f;
 
 			if (warn) {
-				PX4_WARN("Failsafe: Descend with land-speed.");
+				PX4_WARN("Failsafe: stop and wait");
 			}
 
 		} else {
-			// Use the failsafe from the PositionController.
+			// descend with land speed since we can't stop
+			setpoint.thrust[0] = setpoint.thrust[1] = 0.f;
+			setpoint.vz = _param_mpc_land_speed.get();
+
 			if (warn) {
-				PX4_WARN("Failsafe: Descend with just attitude control.");
+				PX4_WARN("Failsafe: blind hover");
+			}
+		}
+
+		if (PX4_ISFINITE(_states.velocity(2))) {
+			// don't move along z if we can stop in all dimensions
+			if (!PX4_ISFINITE(setpoint.vz)) {
+				setpoint.vz = 0.f;
 			}
+
+		} else {
+			// emergency descend with a bit below hover thrust
+			setpoint.vz = NAN;
+			setpoint.thrust[2] = _param_mpc_thr_hover.get() * .8f;
 		}
 
 		_in_failsafe = true;