bpo-34441: Fix ABC.__subclasscheck__ crash on classes with invalid __subclasses__ (GH-8835)


The missing NULL check was reported by Svace static analyzer.
(cherry picked from commit cdbf50cba1664f72ae6621a89c324a32fea70377)

Co-authored-by: Alexey Izbyshev <izbyshev@ispras.ru>
3 files changed