Merge: lib: fix semantic on has_substring on extreme cases

And avoid illegal access outside the string.

Detected thanks to the interpreted that warns on out-of-bound accesses in nativestrings.

Pull-Request: #854
Reviewed-by: Lucas Bajolet <r4pass@hotmail.com>
Reviewed-by: Alexandre Terrasa <alexandre@moz-code.org>

diff --cc lib/standard/string.nit
Simple merge