In PyObject_IsTrue(), don't call function pointers that are NULL

(nb_nonzero, mp_length, sq_length).

Objects/object.c

@@ -469,11 +469,14 @@ PyObject_IsTrue(v)
 	int res;
 	if (v == Py_None)
 		res = 0;
-	else if (v->ob_type->tp_as_number != NULL)
+	else if (v->ob_type->tp_as_number != NULL &&
+		 v->ob_type->tp_as_number->nb_nonzero != NULL)
 		res = (*v->ob_type->tp_as_number->nb_nonzero)(v);
-	else if (v->ob_type->tp_as_mapping != NULL)
+	else if (v->ob_type->tp_as_mapping != NULL &&
+		 v->ob_type->tp_as_mapping->mp_length != NULL)
 		res = (*v->ob_type->tp_as_mapping->mp_length)(v);
-	else if (v->ob_type->tp_as_sequence != NULL)
+	else if (v->ob_type->tp_as_sequence != NULL &&
+		 v->ob_type->tp_as_sequence->sq_length != NULL)
 		res = (*v->ob_type->tp_as_sequence->sq_length)(v);
 	else
 		res = 1;