Fix corner case causing TypeError in userJoin

I believe this issue was caused by a single user in the channel being kicked for a duplicate login, thereby unloading the channel while the for loop was iterating, so this.users became undefined before the loop exited and thus there was a TypeError for reading this.users.length

lib/channel.js

@@ -881,16 +881,16 @@ Channel.prototype.userJoin = function(user, password) {
             Logger.errlog.log("keys: " + Object.keys(this).join(","));
             return;
         }
-        for(var i = 0; i < this.users.length; i++) {
-            if(this.users[i].name.toLowerCase() == user.name.toLowerCase()) {
-                if (this.users[i] == user) {
+        this.users.forEach(function (u) {
+            if(u.name.toLowerCase() == user.name.toLowerCase()) {
+                if (u == user) {
                     Logger.errlog.log("Wat: userJoin() called on user "+
                                       "already in the channel");
-                    break;
+                    return;
                 }
-                this.kick(this.users[i], "Duplicate login");
+                this.kick(u, "Duplicate login");
             }
-        }
+        });
     }
 
     this.users.push(user);