audio: pipewire: Pass proper parameter to user audio callback

The audio callbacks should pass the callbackspec.userdata parameter to the callback, not spec.userdata

Co-authored-by: Oschowa <Oschowa@web.de>

src/audio/pipewire/SDL_pipewire.c

@@ -793,7 +793,7 @@ output_callback(void *data)
   if (!SDL_AtomicGet(&this->paused)) {
     if (!this->stream) {
       SDL_LockMutex(this->mixer_lock);
-      this->callbackspec.callback(this->spec.userdata, dst, this->callbackspec.size);
+      this->callbackspec.callback(this->callbackspec.userdata, dst, this->callbackspec.size);
       SDL_UnlockMutex(this->mixer_lock);
     } else {
       int got;