squash: remove pending tasks from the queue before scheduling close

modules/util/AsyncQueue.js

         this._stopped = false;
     }
 
+    /**
+     * Removes any pending tasks from the queue.
+     */
+    clear() {
+        this._queue.kill();
+    }
+
     /**
      * Internal task processing implementation which makes things work.
      */

modules/xmpp/JingleSessionPC.js

         this.peerconnection.onnegotiationneeded = null;
         this.peerconnection.onsignalingstatechange = null;
 
+        // Remove any pending tasks from the queue
+        this.modificationQueue.clear();
+
         this.modificationQueue.push(finishCallback => {
             // The signaling layer will remove it's listeners
             this.signalingLayer.setChatRoom(null);
             this.peerconnection && this.peerconnection.close();
             finishCallback();
         });
+
+        // No more tasks can go in after the close task
         this.modificationQueue.shutdown();
     }