move divide by zero inside guard

speculative fix to see if this helps developer.
It's the right thing to do in any case.

TBR=reed@google.com

Change-Id: I4fa576a096a6188f290957a7f2fabe73668f142d
Reviewed-on: https://skia-review.googlesource.com/84521
Reviewed-by: Cary Clark <caryclark@skia.org>
Commit-Queue: Cary Clark <caryclark@skia.org>
2 files changed