gh-155376: prevent supercheck NULL dereference

[BHUVANSH855]

Summary

Fix a NULL dereference in supercheck() when an uninitialized super object is used as a descriptor.

What happened?

super.__new__(super) can create a super object whose internal type field is NULL. When that object is subsequently used through __get__(), supercheck() receives the NULL type pointer.

The error-handling path in supercheck() eventually formats an error message using type->tp_name, causing a NULL pointer dereference and a segmentation fault.

Fix

This change:

• Adds an explicit NULL check for type in supercheck().
• Raises TypeError("super object has no type") instead of dereferencing the NULL pointer.
• Uses tp_alloc() directly in super_descr_get() for the internal super object allocation.
• Adds a regression test for the uninitialized super case.
• Adds a NEWS entry documenting the crash fix.

Reproducer

Before this change:

s = super.__new__(super)
s.__get__(1)

resulted in:

Segmentation fault (core dumped)

After this change:

TypeError: super object has no type

Tests

./python -m test test_super — passed
./python -m test test_super test_descr — passed
git diff --check — passed

Closes issue gh-155376