grltrackerwrapper: Fix early playlists change check crash
In 41c60ec7 the playlist wrapper creation gets delayed until after the initial songs list has filled. It is now possible however that on larger collections change tracking is already starting on the songs list before the playlist wrapper is loaded. This can cause a crash when checking for smart playlist changes on an uninitialized GrlTrackerPlaylists object.
Add a check for existence of the GrlTrackerPlaylists object to avoid this.
Fixes: #382 (closed)