[m-rev.] for review: Fix compilation with GCC 13.

[Julien Fischer]

On Tue, 11 Jul 2023, Peter Wang wrote:

> Convince GCC 13 that a sprintf() call will not overflow.
> Fixes GitHub issue #123.
>
> runtime/mercury_ml_expand_body.h:
>    As above.

That's fine.

Julien.