ceph: fix null pointer dereference

When register_session() is given an out-of-range argument for mds,
ceph_mdsmap_get_addr() will return a null pointer, which would be given to
ceph_con_open() & be dereferenced, causing a kernel oops. This fixes bug #4685
in the Ceph bug tracker <http://tracker.ceph.com/issues/4685

>.

Signed-off-by: Nathaniel Yazdani <n1ght.4nd.d4y@gmail.com>
Reviewed-by: Sage Weil <sage@inktank.com>