osd/OSDMap: ignore xinfo if features == 0

osd/OSDMap: ignore xinfo if features == 0

Some old bug (e.g., http://tracker.ceph.com/issues/20751) could
result in an UP+EXISTS osd having features==0.  If that happens,
we shouldn't crash the mon, which (reasonably) does

   if (osdmap.get_epoch()) {
     if (osdmap.get_num_up_osds() > 0) {
       assert(osdmap.get_up_osd_features() & CEPH_FEATURE_MON_STATEFUL_SUB);
       check_subs();
     }
   }

Signed-off-by: Sage Weil <sage@redhat.com>
(cherry picked from commit 8399833f3d221364a8ededb97cd4e1c5a258d924)