Oversized scroller causes snap error

Steps to reproduce:
 - Copy the [event passthrough demo](http://lab.cubiq.org/iscroll5/demos/event-passthrough/)
 - Add ```snap: "li"``` to config
 - Change ```#scroller```width to something higher, e.g. 3400 px
 - Scroll to the very end and try to push the last item out of the screen