add safety check on scroll method

When Scroller is destroyed right after starting to animate the scroll position, it throws an exception. (TypeError: 'undefined' is not an object (evaluating 'data.horizontal') on line 95) Catch if the data attribute is already gone and don’t try to do anything if that’s the case. Tested and confirmed this will prevent the error.

The diff looks really strange because of a block indent, but the only thing *actually* modified is scroller.js line 73.