Avoid div by zero in ProcessorTest to please UBSAN

Change-Id: I1e49ab46975bb8e5eff08bc5afe7ffed1c078309
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/261550
Reviewed-by: Brian Salomon <bsalomon@google.com>
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
1 file changed