Fix null reference bug

lib/channel/playlist.js

@@ -987,6 +987,10 @@ PlaylistModule.prototype.startPlayback = function (time) {
                  * Its purpose is to allow modification of playback data before
                  * users are sent a changeMedia
                  */
+                if (!self.current || !self.current.media) {
+                    return;
+                }
+
                 self.sendChangeMedia(self.channel.users);
                 self.channel.notifyModules("onMediaChange", [self.current.media]);
             }