Caught with -d=checkptr.

Updates #22218.

Change-Id: Ic0fcff4d2c8d83e4e7f5e0c6d01f03c9c7766c6d
Reviewed-on: https://go-review.googlesource.com/c/go/+/201617
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>