lib{c,openbsd}: use ckd_mul() for overflow checking in re(c)allocarray

[fuz]

Summary

This makes the code easier to understand and slightly faster,
but requires C23. calloc() would benefit, too, but I didn't
want to touch the imported jemalloc code base.

[kib]

lib/libopenbsd/recallocarray.c

36

I would wrote

if (ckd_mul(&newsize, newnmemb, size) ||
    ckd_mul(&oldsize, oldnmemb, size)) {
             errno = EINVAL;
             return NULL;
}

since there are much simpler conditions now.

[fuz]

lib/libopenbsd/recallocarray.c

36

This cannot be done as one needs to fail with ENOMEM while the other fails with EINVAL.

[kib]

lib/libopenbsd/recallocarray.c

36

Indeed, missed that.