gh-131181: Fix possible NULL pointer dereference in Modules/_ctypes/_ctypes.c

[dyupina]

In the PyCData_NewGetBuffer() (file Modules/_ctypes/_ctypes.c), the PyObject_stgdict() is called, which may return NULL.
Its result is assigned to dict, then dict is dereferenced (view->format = dict->format ? dict->format : "B";).
If dict is NULL, a null pointer dereference will occur, so a check for NULL need to be added.

Similarly, in the PyCData_reduce() (file Modules/_ctypes/_ctypes.c) PyObject_stgdict(myself) may return NULL, so need to check result to prevent null pointer dereferencing.

Commit dcaf33a (Author: @encukou) fixes this problems, but it is very large and probably difficult to backport, so I suggest adding assert(); before the dereference.

This needs to be added to branches 3.1 - 3.12.
Found by Linux Verification Center (linuxtesting.org) with SVACE.

• Issue: Add some non-NULL assertion checks in Modules/_ctypes/_ctypes.c #131181

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