I noticed this leak while writing CL 214977.

Change-Id: I7566952b8e4bc58939d23435aea86576fc58ddca
Reviewed-on: https://go-review.googlesource.com/c/go/+/214978
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>