Possible race in dns_dispatch_connect()

A test failure turned up in !6562 (merged) that indicates a probable race in dns_dispatch_connect(): thread 1 sets disp->tcpstate from NONE to CONNECTING; thread 2 finds disp->tcpstate is CONNECTING, locks the dispatch, appends something to disp->pending, and unlocks; thread 1 locks the dispatch, finds that disp->pending is not empty, and asserts.

I think the correct fix is just to remove the INSIST that the list has to be empty.