osd/PrimaryLogPG: Avoid accessing destroyed references in finish_degraded_object

osd/PrimaryLogPG: Avoid accessing destroyed references in finish_degraded_object

As follows:
for (auto i = callbacks_for_degraded_object.begin(); i != callbacks_for_degraded_object.end();) {
    finish_degraded_object((i++)->first);
}

void PrimaryLogPG::finish_degraded_object(const hobject_t oid)
{
  if (callbacks_for_degraded_object.count(oid)) {
    contexts.swap(callbacks_for_degraded_object[oid]);
    callbacks_for_degraded_object.erase(oid);   // Release
  }

  map<hobject_t, snapid_t>::iterator i = objects_blocked_on_degraded_snap.find(
    oid.get_head());  // Access
  ...
}

Fixes: https://tracker.ceph.com/issues/41250
Signed-off-by: Tao Ning <ningtao@sangfor.com.cn>