check for unsigned int overflow

Hi Oliver,

We found a crash that occurred when overflowing NSUInteger. When index is 0 and 1 is subtracted we get an overflow and subsequently an index oob when trying to view some content. I'm not sure where to put a test for this but I believe the offending HTML was simply:

    <div dir="auto" style="margin-bottom:12px;"></div>

I'm not positive that HTML triggered the crash because tracking this down was pretty difficult and I don't have a lot to go on. I do know checking for the overflow fixed the issue.