tizspotifysrc: playback stalls when 'container_loaded' is called by spotify

[tizonia]

'container_loaded' is typically called at the beginning of the execution, but sometimes it gets called in the middle of the playlist run, and these cases, the playback stalls. Using prev or next will get it going again.

Currently, 'container_loaded' has the side effect of "reset" the current playlist pointer, which is only works if this is only called once at the beginning. The logic in this callback should check if another playback is in progress and avoid resetting the pointer.