test/issue71226: add cast to avoid clang error

test/issue71226: add cast to avoid clang error

Change-Id: I2d8ecb7b5f48943697d454d09947fdb1817809d0
Reviewed-on: https://go-review.googlesource.com/c/go/+/646295
TryBot-Bypass: Ian Lance Taylor <iant@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>
Auto-Submit: Ian Lance Taylor <iant@golang.org>