Copy the DNS entry going into the cache, to avoid races.

The shuffleAnswers() call ends up acting on the same data,
so it seems simplest to resolve ownership arguments by copying.

nameserver/cache.go

@@ -155,7 +155,7 @@ func (e *cacheEntry) setReply(reply *dns.Msg, ttl int, flags uint8, now time.Tim
 	e.putTime = now
 
 	if reply != nil {
-		e.reply = *reply
+		e.reply = *reply.Copy()
 		e.ReplyLen = reply.Len()
 	}