Error check to prevent exception from Cipher

plugin/multiplex/index.js

@@ -15,6 +15,7 @@ var opts = {
 
 io.sockets.on('connection', function(socket) {
 	socket.on('slidechanged', function(slideData) {
+		if (typeof slideData.secret == 'undefined' || slideData.secret == null || slideData.secret === '') return;
 		if (createHash(slideData.secret) === slideData.socketId) {
 			slideData.secret = null;
 			socket.broadcast.emit(slideData.socketId, slideData);