Noticed and fix by Alex Brainman.

Tested in https://golang.org/cl/23005 (which makes all compiler
warnings fatal during development)

Fixes #15623

Change-Id: Ic19999fce8bb8640d963965cc328574efadd7855
Reviewed-on: https://go-review.googlesource.com/23010Reviewed-by: Alex Brainman <alex.brainman@gmail.com>