Correct overflow detection when going negative.

Updates golang/go#25023

Change-Id: Ic2ddb7ee757f081d1826bfbbfd884e2b7e819335
Reviewed-on: https://go-review.googlesource.com/111675Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>