bpo-44184: Fix subtype_dealloc() for freed type (GH-26274)

Fix a crash at Python exit when a deallocator function removes the
last strong reference to a heap type.

Don't read type memory after calling basedealloc() since
basedealloc() can deallocate the type and free its memory.

_PyMem_IsPtrFreed() argument is now constant.
4 files changed