Fixes JS error on unload

JitsiConference.js

  * Leaves the conference.
  */
 JitsiConference.prototype.leave = function () {
-    if(this.xmpp)
+    if(this.xmpp && this.room)
         this.xmpp.leaveRoom(this.room.roomjid);
     this.room = null;
 };

doc/example/example.js

 }
 
 function unload() {
-//    room.leave();
+   room.leave();
     connection.disconnect();
 }
 

lib-jitsi-meet.js

  * Leaves the conference.
  */
 JitsiConference.prototype.leave = function () {
-    if(this.xmpp)
+    if(this.xmpp && this.room)
         this.xmpp.leaveRoom(this.room.roomjid);
     this.room = null;
 };