There are no strange messages anymore, so define type MyBuffer in the
block where it is used.

Change-Id: Ic65b15dc76a40f6f734d9ac2116338502fbb66fd
Reviewed-on: https://go-review.googlesource.com/107735
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>