add assert around gmi_mcast calls since it can return a failure now.

BKrev: 4154791ch3ag3FHVGxaUfrVFsvN3sQ


git-svn-id: http://svn.fedorahosted.org/svn/corosync/trunk@293 fd59a12c-fef9-0310-b244-a6a79926bd2f