[3.13] gh-143003: Fix possible shared buffer overflow in bytearray.extend() (GH-143086) (GH-143448)

When __length_hint__() returns 0 for non-empty iterator, the data can be
written past the shared 0-terminated buffer, corrupting it.
(cherry picked from commit 522563549a49d28e763635c58274a23a6055f041)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>