[3.13] gh-131998: Fix NULL dereference when using an unbound method descriptor in a specialized code path (GH-132000)

(cherry picked from commit ac3c439)

Co-authored-by: Peter Bierma <zintensitydev@gmail.com>
Co-authored-by: sobolevn <mail@sobolevn.me>
Co-authored-by: Victor Stinner <vstinner@python.org>
Co-authored-by: Mark Shannon <mark@hotpy.org>

Author: 4 people