[3.13] gh-71810: Fix _PyLong_AsByteArray() undefined behavior (GH-138873)

[miss-islington]

Don't read p[-1] when p is an empty string: when n==0.
(cherry picked from commit 8b5ce31)

Co-authored-by: Victor Stinner vstinner@python.org

• Issue: int.to_bytes() and int.from_bytes(): raise ValueError when bytes count is zero #71810