dig +nssearch: send more queries even if sending the previous one fails

dig +nssearch: send more queries even if sending the previous one fails

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.