In the NSSEARCH followup lookup, when one of the queries fails to be
sent, DiG doesn't start the next query. This is a mistake, because in
NSSEARCH mode the queries are independent and DiG shouldn't stop the
lookup process just because sending a query to one of the name servers
returns an error code.
Restructure the `send_done()` function to unconditionally send the next
query in NSSEARCH mode, if it exists.
98da5129a5