[3.14] gh-143602: Fix duplicate buffer exports in io.BytesIO.write (#143629) (#143872)

gh-143602: Fix duplicate buffer exports in io.BytesIO.write (#143629)

Fix an inconsistency issue in io.BytesIO.write() where the buffer was exported
twice, which could lead to unexpected data overwrites and position drift when
the buffer changes between exports.

(cherry picked from commit c461aa99e2fabbaf5859c0a8a93e08306ee8115d)

Co-authored-by: zhong <60600792+superboy-zjc@users.noreply.github.com>