navigator: do not set a mode while disarmed

This avoids premature state machine execution, most notably for missions.

In particular this fixes these issues:
- when landed, and the vehicle has a valid mission: switching to mission
  would execute the first item.
- after executing a mission with a land waypoint, the mission is reset
  and the first item is executed.

In case the first waypoint is a takeoff, it would print a user message:
"Takeoff to x meters above home.". If it was a vehicle command, it would
execute that.
Note that because the vehicle is disarmed, the vehicle would not actually
do anything (except for commands that are accepted while disarmed).

src/modules/navigator/navigator_main.cpp

@@ -647,6 +647,11 @@ Navigator::run()
 			break;
 		}
 
+		// Do not execute any state machine while we are disarmed
+		if (_vstatus.arming_state != vehicle_status_s::ARMING_STATE_ARMED) {
+			navigation_mode_new = nullptr;
+		}
+
 		// update the vehicle status
 		_previous_nav_state = _vstatus.nav_state;