dig +nssearch crashes for the root zone

dig +nssearch . command crashes, as discovered and reported by Thomas Amgarten in this comment.

This is a lookup reference counting bug, happening when dig sees a bad cookie in one of the queries running in parallel, and re-queues the lookup.

This isn't strictly related to the root zone, but the root zone has quite a few name servers, which increases the chances of encountering a ;; BADCOOKIE, retrying. situation.

I'm preparing an MR with a fix and a more detailed description of the bug.