pythongh-96821: Fix undefined behaviour in _testcapimodule.c (pythonGH-96915)

* pythongh-96821: Assert for demonstrating undefined behaviour

* Fix UB

Co-authored-by: C.A.M. Gerlach <CAM.Gerlach@Gerlach.CAM>
(cherry picked from commit cbdeda8)

Co-authored-by: Matthias Görgens <matthias.goergens@gmail.com>

Author: matthiasgoergens

Misc/NEWS.d/next/Core and Builtins/2022-09-18-08-47-40.gh-issue-96821.Co2iOq.rst

@@ -0,0 +1 @@
+Fix undefined behaviour in ``_testcapimodule.c``.

Modules/_testcapimodule.c

@@ -5475,8 +5475,10 @@ meth_fastcall_keywords(PyObject* self, PyObject* const* args,
     if (pyargs == NULL) {
         return NULL;
     }
+    assert(args != NULL || nargs == 0);
+    PyObject* const* args_offset = args == NULL ? NULL : args + nargs;
     PyObject *pykwargs = PyObject_Vectorcall((PyObject*)&PyDict_Type,
-                                              args + nargs, 0, kwargs);
+                                              args_offset, 0, kwargs);
     return Py_BuildValue("NNN", _null_to_none(self), pyargs, pykwargs);
 }