The `send_done()` callback needs to access query's `link.next` pointer
when running in `+nssearch` mode, even if the query is already canceled
or serviced, which can happen when `recv_done()` happens to be called
earlier than `send_done()`.
Keep the next query's pointer before unlinking the query from the
lookup's queries list in `clear_query()` so that `send_done()` can
use it even if the query is cleared.
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