The TCP dispatches are removed from the dispatchmgr->list in the
dispatch_destroy() and there's a brief period of time where
dns_dispatch_gettcp() can find a dispatch in connected state that's
being destroyed.
Set the dispatch state to DNS_DISPATCHSTATE_NONE in the TCP connection
callback if there are no responses waiting, and ignore TCP dispatches
with zero references in dns_dispatch_gettcp().
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