Use Py_CLEAR(). Add unrelated test.

Lib/test/test_set.py

@@ -23,6 +23,9 @@ class TestJointOps(unittest.TestCase):
         self.s = self.thetype(word)
         self.d = dict.fromkeys(word)
 
+    def test_new_or_init(self):
+        self.assertRaises(TypeError, self.thetype, [], 2)
+
     def test_uniquification(self):
         actual = sorted(self.s)
         expected = sorted(self.d)

Objects/setobject.c

@@ -105,7 +105,7 @@ frozenset_dict_wrapper(PyObject *d)
 	w = (PySetObject *)make_new_set(&PyFrozenSet_Type, NULL);
 	if (w == NULL)
 		return NULL;
-	Py_DECREF(w->data);
+	Py_CLEAR(w->data);
 	Py_INCREF(d);
 	w->data = d;
 	return (PyObject *)w;