Also fix behavior of Buffers.WriteTo when writev returns an error.

Fixes #16266.

Change-Id: Idc9503408ce2cb460663768fab86035cbab11aef
Reviewed-on: https://go-review.googlesource.com/31584
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>