Prevent playing prev_track if there are no tracks.

src/playlist.cpp

@@ -46,7 +46,7 @@ bool Playlist::prev_track() {
 	} else {
 		set_current_position(current_file - 1, 0);
 	}
-	return true;
+	return files.size()>0;
 }
 
 void Playlist::restart() {