Fix segfault during membership changes with multiple leaves
at the same time.

BKrev: 40eaeec3CJrXaOzED0qddXmUUurQdg


git-svn-id: http://svn.fedorahosted.org/svn/corosync/trunk@50 fd59a12c-fef9-0310-b244-a6a79926bd2f