[3.13] gh-141311: Avoid assertion in BytesIO.readinto() (GH-141333) (GH-141478)

Fix error in assertion which causes failure if pos is equal to PY_SSIZE_T_MAX.
Fix undefined behavior in read() and readinto() if pos is larger that the size
of the underlying buffer.
(cherry picked from commit 7d54374f9c7d91e0ef90c4ad84baf10073cf1d8a)