Fix GroupSetPermissionAccessEntity property load

Fix `GroupSetPermissionAccessEntity::getAccessEntityUsers()` accessing
of unset `groupset` property

- Check if `$this->groupset` is set and attempt to load it if not
  set by calling `GroupSetPermissionAccessEntity::load()`

Of note is that this could still fail with a fatal error if the gID
does not exist and the `load()` method still does not set `groupset`
Merge pull request #1533 from mkly/fix_group_set_permission_access_entity_2

Fix GroupSetPermissionAccessEntity property load