Fix ImageFilter fuzzer issue

What appears to be happening in this fuzz is that a paint index inside the picture of an SkPictureImageFilter is getting changed to be out of range.

BUG=skia:5192
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1893423002

Review URL: https://codereview.chromium.org/1893423002
7 files changed