cmd/fix: fix cast check

Need 2-result cast so we can check the result correctly.

Fixes #23762

Change-Id: Icac3a5415156fe918988f369d6022a9a29c14089
Reviewed-on: https://go-review.googlesource.com/93078


Run-TryBot: Keith Randall <khr@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>