[library] Finish scroller when reseting state

Fixes crash when adapter gets replaced/removed while performing a fling.

Steps to reproduce:

1. Use an adapter with enough items to fling for a while.
2. Replace adapter or set to `null` while performing fling.
3. Boom shakalaka

![Boom shakalaka](http://i52.tinypic.com/2d82fz7.gif)