service/audio/audout_u: Only actually stop the audio stream in StopAudioOut if the stream is playing

The service itself only does further actions if the stream is playing.
If the stream is already stopped, then it just exits successfully.

src/core/hle/service/audio/audout_u.cpp

@@ -107,7 +107,9 @@ private:
     void StopAudioOut(Kernel::HLERequestContext& ctx) {
         LOG_DEBUG(Service_Audio, "called");
 
-        audio_core.StopStream(stream);
+        if (stream->IsPlaying()) {
+            audio_core.StopStream(stream);
+        }
 
         IPC::ResponseBuilder rb{ctx, 2};
         rb.Push(RESULT_SUCCESS);