Leaves room only when it is already joined.

In case of hitting errors like max participant limit reached and when clicking hangup, the attempt to leave room second time results error and reload screen.

conference.js

     leaveRoomAndDisconnect() {
         APP.store.dispatch(conferenceWillLeave(room));
 
-        return room.leave().then(disconnect, disconnect);
+        if (room.isJoined()) {
+            return room.leave().then(disconnect, disconnect);
+        }
+
+        return disconnect();
     },
 
     /**