CRM_Member_BAO_MembershipBlock - Fix test crash

See also:

https://github.com/eileenmcnaughton/civicrm-core/commit/4d5c2eb510a6be0017085d1f8dae09495e28e74b#commitcomment-3203066

CRM/Member/BAO/MembershipBlock.php

+<?php
+/*
+ +--------------------------------------------------------------------+
+ | CiviCRM version 4.3                                                |
+ +--------------------------------------------------------------------+
+ | Copyright CiviCRM LLC (c) 2004-2013                                |
+ +--------------------------------------------------------------------+
+ | This file is a part of CiviCRM.                                    |
+ |                                                                    |
+ | CiviCRM is free software; you can copy, modify, and distribute it  |
+ | under the terms of the GNU Affero General Public License           |
+ | Version 3, 19 November 2007 and the CiviCRM Licensing Exception.   |
+ |                                                                    |
+ | CiviCRM is distributed in the hope that it will be useful, but     |
+ | WITHOUT ANY WARRANTY; without even the implied warranty of         |
+ | MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.               |
+ | See the GNU Affero General Public License for more details.        |
+ |                                                                    |
+ | You should have received a copy of the GNU Affero General Public   |
+ | License and the CiviCRM Licensing Exception along                  |
+ | with this program; if not, contact CiviCRM LLC                     |
+ | at info[AT]civicrm[DOT]org. If you have questions about the        |
+ | GNU Affero General Public License or the licensing of CiviCRM,     |
+ | see the CiviCRM license FAQ at http://civicrm.org/licensing        |
+ +--------------------------------------------------------------------+
+*/
+
+/**
+ *
+ * @package CRM
+ * @copyright CiviCRM LLC (c) 2004-2013
+ * $Id$
+ *
+ */
+class CRM_Member_BAO_MembershipBlock extends CRM_Member_DAO_MembershipBlock {
+  // placeholder required because there's some magic code which assumes that BAOs exist
+}
\ No newline at end of file