PNG: Scanlines + String.sub out of bounds

[cfcs]

(this is part of @olleolleolle + my work on a test suite for imagelib)

Here we reach a Fatal error: exception Invalid_argument("String.sub / Bytes.sub") in
https://github.com/rlepigre/ocaml-imagelib/blob/master/src/imagePNG.ml#L884

Trigger file:

(length is also typo'ed as lenght in the paragraph above, so if whoever fixes this would be kind enough to correct that as well, that would be nice :) )