Prevents a memory leak.

Tests (to be updated) in Go standard library.

Updates golang/go#14084

Change-Id: I3ff602a013bb8fda7a17bccb31beadb08421ae6a
Reviewed-on: https://go-review.googlesource.com/19134Reviewed-by: Blake Mizerany <blake.mizerany@gmail.com>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>