Thanks to Andy Bursavich for the example attack.
Pair programmed with Dmitri Shuralyov.

Fixes golang/go#12843

Change-Id: Ic412c9364b37a10e5164232aa809b956874fae08
Reviewed-on: https://go-review.googlesource.com/15751Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>