before this change, when io.MultiReader was called many times but contain few
underlying readers, calls to Read were unnecessarily expensive.

Fixes #13558

Change-Id: I3ec4e88c7b50c075b148331fb1b7348a5840adbe
Reviewed-on: https://go-review.googlesource.com/17873Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>