Found by cmd/vet.

Change-Id: Id570ecd76c3f1efd9696680ccd9799610217f8f7
Reviewed-on: https://go-review.googlesource.com/17042
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>