Patch from Russ.

No bug identified, but I didn't search exhaustively. The new code is
easier to read.

Fixes #13621

Change-Id: Ifda936e4101116fa254ead950b5fe06adb14e977
Reviewed-on: https://go-review.googlesource.com/17981Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>