Fixes molnarg#228

In the case where this._push(frame) returns null (i.e., the frame is too
large for the window and split or the window size is <=0), moreNeeded
will be set to null. Then this._queue.push(frame) is called, but
moreNeeded is still null. Thus, any time the window is <=0 or the frame
is split we'll hit the assert:

  var moreNeeded = null;
  if (this._queue.length === 0) {
    moreNeeded = this._push(frame);
  }

  if (moreNeeded === null) {
    this._queue.push(frame);
  }

  return moreNeeded;

Credit goes to @jrabek for original version of this patch