0 byte reads at EOF weren't returning EOF.

Change-Id: I19b5fd5a72e83d49566a230ce4067be03f00d14b
Reviewed-on: https://go-review.googlesource.com/22740Reviewed-by: Bryan Mills <bcmills@google.com>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>