flow/conn: Fix data race in message pipe.
flow/conn: Fix data race in message pipe.

This race can happen if the context is canceled while the the conn
is being established.

Change-Id: Icea14d616791956de30da720d1b7d624b0e629b7

vanadium/go.ref,vanadium/go.ref,vanadium/go.ref,vanadium/go.ref,vanadium/go.ref,vanadium/go.ref