9cdec11346
The watchdog and the end of the call race by construction: a cancellation landing as the reply arrives closes a conn the call had already finished with, and c.conn still pointed at the closed socket. It was survivable before this. A write to a closed socket is errWriteLost, which re-dials and retries, and that retry is safe because nothing was sent. So this buys one round trip, not a correctness fix, and the new test says so rather than pretending to catch a break.