scan: for style, adjust code for bad scan read counts

Make the code more consistent with the rest of the file.
Should have caught this in review of CL 225357.

Change-Id: I12824cb436539c31604684e043ebb7587cc92471
Reviewed-on: https://go-review.googlesource.com/c/go/+/225557


Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>