[3.13] gh-71810: Fix _PyLong_AsByteArray() undefined behavior (GH-138873) (#138884)

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

Don't read p[-1] when p is an empty string: when n==0.
(cherry picked from commit 8b5ce31c2b44d9bf82e6119e90a52dd530bfd1db)

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