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

(cherry picked from commit ac3c439)

Co-authored-by: Peter Bierma <[email protected]>
Co-authored-by: sobolevn <[email protected]>
Co-authored-by: Victor Stinner <[email protected]>
Co-authored-by: Mark Shannon <[email protected]>

Author: 4 people