Support root requests with EDNS0 OPT records

Support root requests with EDNS0 OPT records

A DNS request with a 0 length QNAME and an additional record (e.g. an
EDNS0 one) is a valid one. A root nameservers request sent by a client
requesting for EDNS0 is one of those.
So label_count can potentially be left at 0 with the EDNS0 record still
to be parsed. In that case label_ptr should just be moved one byte forward,
instead of staying at NULL. The latter will cause segmentation fault when
trying to dereference it from the EDNS0 record parsing code.