Picks up vet fix from CL 176357.

Change-Id: Ia77cd4a582c4edfbe59bbc311e6ce14046df0e83
Reviewed-on: https://go-review.googlesource.com/c/go/+/177137
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>