upstream: when receving a file in sink(), be careful to send at

most a single error response after the file has been opened. Otherwise the
source() and sink() can become desyncronised. Reported by Daniel Goujot,
Georges-Axel Jaloyan, Ryan Lahfa, and David Naccache.

ok deraadt@ markus@

OpenBSD-Commit-ID: 6c14d233c97349cb811a8f7921ded3ae7d9e0035