Fix play bleeding into next track after auto stop

If "fade out on stop" is enabled, the "stop after this track" feature
would stop not stop the on current track, but instead start playing
the next track and fade out on that immediately. This patch disables
fadeout when the engine is stopped by HandleStopAfter().
Merge pull request #4308 from TheUbuntuGuy/master

Fix play bleeding into next track after auto stop