DUFFZERO was disabled due to issue #12108. CL 27592 fixed it and
enabled DUFFZERO in general, but this one was forgotten.

Change-Id: I0476a3a0524c7b54218f7a747bdba76cd823fbc5
Reviewed-on: https://go-review.googlesource.com/32181
Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>