[3.13] gh-130567: Fix possible crash in locale.strxfrm() (GH-138940) (GH-139266)

On some macOS versions there was an off-by-one error in wcsxfrm() which
caused writing past the end of the array if its size was not calculated
by running wcsxfrm() first.
(cherry picked from commit 5854cf38a25ab8b0c6ab0296098166014f77caa3)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>
Co-authored-by: Ronald Oussoren <ronaldoussoren@mac.com>