cache insert null message when can't connect dns server

[foreversunyao]

forwarding.go

if absoluteErr != nil {
log.Errorf("[%d] Error looking up literal qname '%s' with upstreams: %v", req.Id, name, absoluteErr)
return nil
}

if not return nil here, cache will keep null message, and when dns server works dig will get null message from the cache.