Fix root domain name parsing

This fixes a query for the root zone (issued from [`native-dns`](https://github.com/tjfontaine/node-dns)), like this:

```js
var question = dns.Question({
    name: "",
    type: "NS",
});
```
Previously, the returning packet to this query could not be parsed and `native-dns` never received the packet and ran into a timeout.

From [RFC 1035](https://www.ietf.org/rfc/rfc1035.txt): The root domain name is defined by a single octet of zeros at 92; the root domain name has no labels.