Avoid out-of-bounds access to grid_points

Bug: oss-fuzz:6521
Change-Id: I03ce010633ad340c0b5f78cc2f3f16ae199a9240
Reviewed-on: https://skia-review.googlesource.com/109800
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
3 files changed