Fix error: Cannot read property 'top' of undefined

Lines [1499](https://github.com/ivaynberg/select2/blob/d487fc58a889b471f7d587cc61baeca1d68de633/select2.js#L1499) and [1509](https://github.com/ivaynberg/select2/blob/d487fc58a889b471f7d587cc61baeca1d68de633/select2.js#L1509) make a call to `child.offset().top`. I don't know what's causing `child.offset()` to return undefined, but it is and it's breaking Select2 so that I can't select the last item in the list (this error occurs when I try hovering over or clicking on the last item in a list).

I can try and create a reduced case, but this fix is 100% backward compatibe, passes all the tests you have for the project and has a 0% chance of breaking anyone's working code. With this patch, my dropdown works just fine. At worst, this does *nothing* for people, at best, it fixes a rare edge case. Like my edge case. And I'm a pretty important person to me.