CL 56950 correctly identified code with checks that were impossible.
But instead of correcting the checks it deleted them.
This CL corrects the code to check what was meant.

Change-Id: Ic89222184ee4fa5cacccae12d750601a9438ac8d
Reviewed-on: https://go-review.googlesource.com/78113
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>