Add debug warning if music player starts pulling tricky shit again

src/main/java/moe/nekojimi/chords/QueueManager.java

@@ -42,8 +42,11 @@ public class QueueManager implements Consumer<Song>
     {
         jukeboxQueue.add(t);
 
-        if (!handler.isPlaying())
+        if (!handler.isPlaying() || handler.getCurrentSong() == null)
             handler.playNext();
+
+        if (handler.isPlaying() != (handler.getCurrentSong() == null))
+            System.err.println("WARNING: handler isPlaying violates contract! Something dumb going on!");
     }
 
     /**