avoid divide by zero

R=kjlubick@google.com
Bug: skia:7623
Change-Id: Ia9fb31201a39a4540d94f97e8d2f06eb290ab6e5
Reviewed-on: https://skia-review.googlesource.com/114082
Commit-Queue: Cary Clark <caryclark@skia.org>
Commit-Queue: Cary Clark <caryclark@google.com>
Reviewed-by: Kevin Lubick <kjlubick@google.com>
1 file changed