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.
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