Prevent double userLeave

lib/channel.js

@@ -860,6 +860,7 @@ Channel.prototype.userJoin = function(user) {
 }
 
 Channel.prototype.userLeave = function(user) {
+    user.channel = null;
     // Their socket might already be dead, so wrap in a try-catch
     try {
         user.socket.leave(this.name);