nanobench: quiet a UBSAN failure div/by/zero

Change-Id: I63cf5ff9572bbe0594f3aa02a9b64e94c3849fc2
Reviewed-on: https://skia-review.googlesource.com/c/183980
Commit-Queue: Hal Canary <halcanary@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
Auto-Submit: Hal Canary <halcanary@google.com>
Reviewed-by: Mike Klein <mtklein@google.com>
1 file changed