Found by upcoming cmd/vet change.

Change-Id: I7a8264a304b2a4f26f3bd418c1b28cc849889c9b
Reviewed-on: https://go-review.googlesource.com/83835
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>