[libc++] Fix bug in ranges::advance

In `ranges::advance(iter, n, bound)`, we'd incorrectly handle the case
where bound < iter and n is 0:

    int a[10];
    int *p = a+5;
    int *bound = a+3;
    std::ranges::advance(p, 0, bound);
    assert(p - a == 5); // we'd return 3 before this patch

This was caused by an incorrect handling of 0 inside __magnitude_geq.

Differential Revision: https://reviews.llvm.org/D117240