Reader fails to detect truncated streams since calls to io.ReadFull
do not check if the error is io.EOF.

Change-Id: I052cd03161e43fec17e3d328106c40e17923e52b
Reviewed-on: https://go-review.googlesource.com/14832Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>