This fixes some 40 warnings from go vet.

Fixes #16134.

Change-Id: Ib9fcba275fe692f027a2a07b581c8cf503b11087
Reviewed-on: https://go-review.googlesource.com/24287Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>