Case happens if Read is called after it has already returned an error
previously. Verified that the new TestPipeCloseWithError test fails
before this change but passes after.

Updates golang/go#20501

Change-Id: I636fbb194f2d0019b0722556cc25a88da2d18e13
Reviewed-on: https://go-review.googlesource.com/44330
Run-TryBot: Tom Bergan <tombergan@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>