Bug fix: Negative startPoint at -setPagesAtContentOffset:

Scrolling to left at index zero causes to `startPoint` be negative.

So `endIndex` ends up with `[_cells count]` value, calling `[self setPageAtIndex:i]` `[_cells count]` times! (since `startIndex` is 0)

I was having a performance problem with `-setPageAtIndex:` being called over 300 times...
Hope this fixes it