Found by vet.

Change-Id: I556d87f853a734002f779b04ba5a3588a3117106
Reviewed-on: https://go-review.googlesource.com/28958
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>