limit vehicle_command subscription updates per cycle

- this is a precaution to eliminate the possibility of getting stuck in
a loop trying to keep up with a high rate publication that could be
coming from a higher priority task

src/modules/navigator/navigator_main.cpp

@@ -223,7 +223,10 @@ void Navigator::run()
 		_home_pos_sub.update(&_home_pos);
 
 		// Handle Vehicle commands
-		while (_vehicle_command_sub.updated()) {
+		int vehicle_command_updates = 0;
+
+		while (_vehicle_command_sub.updated() && (vehicle_command_updates < vehicle_command_s::ORB_QUEUE_LENGTH)) {
+			vehicle_command_updates++;
 			const unsigned last_generation = _vehicle_command_sub.get_last_generation();
 
 			vehicle_command_s cmd{};