pipewire: Don't mark a device disconnected if pw_stream_dequeue_buffer fails.

Apparently this _can_ happen under load, or maybe some other weird condition.

Hopefully this will encourage PipeWire to fire output_callback again, and
we'll just try again later.

Reference Issue #14916.

src/audio/pipewire/SDL_pipewire.c

@@ -976,18 +976,18 @@ static void initialize_spa_info(const SDL_AudioSpec *spec, struct spa_audio_info
 
 static Uint8 *PIPEWIRE_GetDeviceBuf(SDL_AudioDevice *device, int *buffer_size)
 {
-    // See if a buffer is available. If this returns NULL, SDL_PlaybackAudioThreadIterate will return false, but since we own the thread, it won't kill playback.
+    // See if a buffer is available. If this sets *buffer_size=0, then SDL_PlaybackAudioThreadIterate will skip this iteration but try again next time.
-    // !!! FIXME: It's not clear to me if this ever returns NULL or if this was just defensive coding.
-
     struct pw_stream *stream = device->hidden->stream;
     struct pw_buffer *pw_buf = PIPEWIRE_pw_stream_dequeue_buffer(stream);
     if (pw_buf == NULL) {
+        *buffer_size = 0;
         return NULL;
     }
 
     struct spa_buffer *spa_buf = pw_buf->buffer;
     if (spa_buf->datas[0].data == NULL) {
         PIPEWIRE_pw_stream_queue_buffer(stream, pw_buf);
+        *buffer_size = 0;
         return NULL;
     }