(sig is unsigned, so sig-1 >= 0 is always true.)

Fixes #11281.

Change-Id: I4b9d784da6e3cc80816f2d2f7228d5d8a237e2d5
Reviewed-on: https://go-review.googlesource.com/17457Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>