CRM_Member_BAO_MembershipBlock - Fix test crash

See also:

https://github.com/eileenmcnaughton/civicrm-core/commit/4d5c2eb510a6be0017085d1f8dae09495e28e74b#commitcomment-3203066
Merge pull request #729 from totten/fix-membershipblock-error

CRM_Member_BAO_MembershipBlock - Fix test crash