This is not a functional change. nr is a uint64 and can never be less
than zero, remove the no-op comparison.

Fixes #11279

Change-Id: Iebb36cc8fe97428b503e65d01b5e67d2b2bc7369
Reviewed-on: https://go-review.googlesource.com/13876
Run-TryBot: Todd Neal <todd@tneal.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>