named crashes on shutdown after load rpz failed

named crashes on shutdown after load rpz failed

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.