Fix fuzzer-found deserialization bugs

This fixes deserialization bugs found by fuzzing SkPaintImageFilter.

BUG=576908,576910
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1589533002

Review URL: https://codereview.chromium.org/1589533002
4 files changed