`indexOf'` gives incorrect result for empty prefix when starting index is out of bounds

[LiamGoodacre]

The expression indexOf' "" 2 "ab" evaluates to Just 0, however it should be Nothing because the string "ab" doesn't have a character at index 2.

[garyb]

This is fixed by your changes now, isn't it?

[LiamGoodacre]

Yup 😄, fixed by #41
I'd forgotten that I filed this issue.

Thanks for merging!