allow divide by zero

Bug: oss-fuzz:8214
Change-Id: Ieb2e61b23f88e399df3bfb284742d4604b7d27fa
Reviewed-on: https://skia-review.googlesource.com/127322
Reviewed-by: Florin Malita <fmalita@chromium.org>
Commit-Queue: Mike Reed <reed@google.com>
1 file changed