Otherwise, we can end up with the flag set when the timeout is
actually disabled, leading to misbehavior.  Commit
f8e5f156 introduced this bug.

Reported by Peter Eisentraut.  Analysis and fix by Thomas Munro,
tweaked by me.

Discussion: http://postgr.es/m/6a909374-2602-7136-8c70-397330a418f3@2ndquadrant.com