In golang/go#22880, our http2 server was sending a HEADERS response
without a :status header. Our client code correctly returned an error
from RoundTrip, but we forgot to clean up properly, and then
subsequently crashed on a DATA frame.

This fixes the Transport crash. A fix for the server bug will come separately.

Change-Id: Iea3bcf4a8c95963c8b5e2b6dd722177607bd1bc1
Reviewed-on: https://go-review.googlesource.com/80056
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Tom Bergan <tombergan@google.com>