Roaring64 iterator cannot come back from the edge

[Dr-Emann]

Something like this should work

roaring64_bitmap_t *b = roaring64_bitmap_of(1, 100ULL);
roaring64_iterator_t *it = roaring64_iterator_create(b);
// it points at the first item, 100

roaring64_iterator_previous(it);
// it now points before the first item

roaring64_iterator_advance(it);
assert(roaring64_iterator_has_value(it));
assert(roaring64_iterator_value(it) == 100);

But instead it fails, right at the beginning of roaring64_iterator_advance:

if (it->art_it.value == NULL) {
    return (it->has_value = false);
}

In fact, because of this check, it seems like it should be safe to call roaring64_iterator_advance repeatedly even after reaching the end of the bitmap.

Originally posted by @Dr-Emann in #558 (comment)