DNS: fix name length and label count for a root server

Bug: 10674
Change-Id: I44297bfa64c31ab53d2aa61b6e54253b7a5a1e27
Reviewed-on: https://code.wireshark.org/review/5167
Reviewed-by: Pascal Quantin <pascal.quantin@gmail.com>
Tested-by: Pascal Quantin <pascal.quantin@gmail.com>
Reviewed-by: Anders Broman <a.broman58@gmail.com>