output drivers: init SmartLock after exit_and_cleanup

This fixes an invalid memory access when exiting the module:
exit_and_cleanup destroys the object, but lock_guard is destructed after
and accesses the lock.

src/drivers/px4io/px4io.cpp

@@ -525,8 +525,6 @@ void PX4IO::updateFailsafe()
 
 void PX4IO::Run()
 {
-	SmartLock lock_guard(_lock);
-
 	if (should_exit()) {
 		ScheduleClear();
 		_mixing_output.unregister();
@@ -535,6 +533,8 @@ void PX4IO::Run()
 		return;
 	}
 
+	SmartLock lock_guard(_lock);
+
 	perf_begin(_cycle_perf);
 	perf_count(_interval_perf);