This may happen when loading an RPZ failed and the code path skips
calling dns_db_endload().  The dns_rpz_zone_t object is still kept
marked as having registered db.  So when this object is finally
destroyed in rpz_detach(), this code will incorrectly call
`dns_db_updatenotify_unregister()`:

   if (rpz->db_registered)
     dns_db_updatenotify_unregister(rpz->db,
                                    dns_rpz_dbupdate_callback, rpz);

and trigger this assertion failure:

   REQUIRE(db != NULL);

To fix this, only call `dns_db_updatenotify_unregister()` when
`rpz->db` is not NULL.