Avoid divide by zero and infinite loop in approximation code

Bug: oss-fuzz:7214 oss-fuzz:7215
Change-Id: I55f900b0843f834455494b2d45ff190594296150
Reviewed-on: https://skia-review.googlesource.com/117361
Reviewed-by: Mike Klein <mtklein@chromium.org>
Commit-Queue: Brian Osman <brianosman@google.com>
1 file changed