[3.13] gh-127208: Reject null character in _imp.create_dynamic() (GH-127400) (#127418)

gh-127208: Reject null character in _imp.create_dynamic() (GH-127400)

_imp.create_dynamic() now rejects embedded null characters in the
path and in the module name.
(cherry picked from commit b14fdadc6c620875a20b7ccc3c9b069e85d8557a)

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