If the server returns a DATA frame before a HEADERS frame, the client
must close the connection with a PROTOCOL_ERROR as describe in the
section 8.1.2.6.

The current implementation does not reject such sequence and panics
trying to write on the non-initialized bufPipe.

Fixes golang/go#21466

Change-Id: I55755dba259d026529a031e9ff82c915f5e4afae
Reviewed-on: https://go-review.googlesource.com/56770Reviewed-by: Tom Bergan <tombergan@google.com>
Run-TryBot: Tom Bergan <tombergan@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>