mds: fix null pointer deref in resolve on mostly-empty mds
mds: fix null pointer deref in resolve on mostly-empty mds

We may not have any dirfrags during resolve if the mds crashed
while stopping.

ajnelson/ceph,ajnelson/ceph,ajnelson/ceph,ajnelson/ceph,ajnelson/ceph,ajnelson/ceph