[m-rev.] for review: Make MR_utf8_get return error for more ill-formed sequences.

[Julien Fischer]

On Thu, 12 Sep 2019, Peter Wang wrote:

> runtime/mercury_string.c:
>    Make MR_utf8_get, MR_utf8_get_mb return an error for code unit
>    sequences that encode a surrogate code point, or a code point
>    greater than U+10FFFF.
>
> runtime/mercury_string.h:
>    Adjust some comments.

That's fine.

Julien.