MDSMonitor.cc: fix assertion caused by MDSMap::STATE_DNE state

Signed-off-by: Yan, Zheng <zyan@redhat.com>
Merge pull request #3076 from ceph/wip-mon-mdsdne

MDSMonitor.cc: fix assertion caused by MDSMap::STATE_DNE state

Reviewed-by: Greg Farnum <gfarnum@redhat.com>