Only dispatch a callback if it is non-null.

BKrev: 4241cdbcbgCCwsZLJEXicZZTvGpuGQ


git-svn-id: http://svn.fedorahosted.org/svn/corosync/trunk@549 fd59a12c-fef9-0310-b244-a6a79926bd2f