The check_if_done() function can pass control back out to
dighost_shutdown() (which is part of dig.c, host.c, or nslookup.c),
and calling that twice can cause unexpected problems, if it is not
designed to be idempotent.
Since cancel_lookup() calls check_if_done() implicitly, don't call
check_if_done() again when 'next' is NULL.
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