byte is unsigned so the comparison against zero is always true.

Change-Id: I8fa60245972be362ae920507a291f92c0f9831ad
Reviewed-on: https://go-review.googlesource.com/13941
Run-TryBot: Todd Neal <todd@tneal.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>