little refactor - use listener.listener

I was running into a bug with using `return true`. Bizarrely, `listener = listeners[key][i];` returned a listener, but within the conditional `listeners[key][i]` was undefined. changing it to `listener.listener` resolve the matter.

My test case looked something like

``` js
ee.addListener('baz', function() {
  count++;
  ee.addListener('baz', function() {
    count++;
  });
  return true;
});
```

I wasn't able to reproduce the error with a test case, so this might not even be worth it. But I felt like I should at least document it.