fix unsigned compare against zero

Change-Id: Ia97c4d653331414a05e73945d3a9c6a953ffc97b
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/314497
Commit-Queue: Herb Derby <herb@google.com>
Commit-Queue: Michael Ludwig <michaelludwig@google.com>
Auto-Submit: Herb Derby <herb@google.com>
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
1 file changed