http2/h2i: do not assume that stdin value is always 0 Fixes golang/go#17661 Change-Id: Iff004af969c694dde9bc3c56add3a419ab1beac7 Reviewed-on: https://go-review.googlesource.com/32373 Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org> Run-TryBot: Alex Brainman <alex.brainman@gmail.com> TryBot-Result: Gobot Gobot <gobot@golang.org>