GH-96678: Fix undefined behavior in ceval.c (#96708)

Misc/NEWS.d/next/Core and Builtins/2022-09-09-13-13-27.gh-issue-96678.vMxi9F.rst

+Fix case of undefined behavior in ceval.c

Python/ceval.c

@@ -5532,7 +5532,13 @@ initialize_locals(PyThreadState *tstate, PyFunctionObject *func,
     /* Pack other positional arguments into the *args argument */
     if (co->co_flags & CO_VARARGS) {
         PyObject *u = NULL;
+        if (argcount == n) {
+            u = Py_NewRef(&_Py_SINGLETON(tuple_empty));
+        }
+        else {
+            assert(args != NULL);
             u = _PyTuple_FromArraySteal(args + n, argcount - n);
+        }
         if (u == NULL) {
             goto fail_post_positional;
         }