Fix sort changelist for objects with sortable fk

Loading the test app '/admin/app/project/sort/' changelist failed with
the following error:

UnboundLocalError: local variable 'objects' referenced before assignment

(utils.py, check_model_is_sortable)

See test sample_app test_adminsortable_change_list_view_loads_with_sortable_fk

adminsortable/utils.py

@@ -30,5 +30,5 @@ def check_model_is_sortable(cls):
                 objects = cls.model.objects
             else:
                 objects = cls.objects
-        return get_is_sortable(objects.all())
+            return get_is_sortable(objects.all())
     return False