Fix TypeError due to race condition

lib/channel/playlist.js

@@ -1012,6 +1012,10 @@ 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]);