Fix for SF bug 528132 (Armin Rigo): classmethod().__get__() segfault

The proper fix is not quite what was submitted; it's really better to
take the class of the object passed rather than calling PyMethod_New
with NULL pointer args, because that can then cause other core dumps
later.

I also added a testcase for the fix to classmethods() in test_descr.py.

I've already applied this to the 2.2 branch.

Objects/funcobject.c

@@ -474,6 +474,8 @@ cm_descr_get(PyObject *self, PyObject *obj, PyObject *type)
 				"uninitialized classmethod object");
 		return NULL;
 	}
+	if (type == NULL)
+		type = (PyObject *)(obj->ob_type);
  	return PyMethod_New(cm->cm_callable,
 			    type, (PyObject *)(type->ob_type));
 }