http2: allow StreamDep of 0 in HEADERS and PRIORITY frames

But still check that the high bit isn't set.

Fixes golang/go#15444

Change-Id: Ic9339244d3d0c4e4d6573a691b3ba37c9eac3981
Reviewed-on: https://go-review.googlesource.com/22476
Reviewed-by: Andrew Gerrand <adg@golang.org>
2 files changed