Prevent NRE after manual re-detecting the targetElement on iOS

Fix for regression introduced by fix for #57. In certain edge cases (noticed when scrolling down) the second argument of `document.elementFromPoint` can be negative, which causes NRE here: https://github.com/ftlabs/fastclick/blob/master/lib/fastclick.js#L505

So I'd suggest the fallback to original targetElement here.

Unfortunately I cannot provide a good test for it since I was not able to reproduce it in 100% cases. The issue is reproduced intermittently on pages with multiple `<input>` elements.