http2: fix Server race

With Tom Bergan.

Updates golang/go#20704

Change-Id: Ib71202801f8c72af2f22865899c93df1f3753fdd
Reviewed-on: https://go-review.googlesource.com/46008
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Tom Bergan <tombergan@google.com>
2 files changed