fix fuzz

Extreme inputs trigger asserts intended for in range data.
Return an error without asserting unless the test signals
otherwise.

TBR=reed@google.com
BUG=630736
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2178923002

Review-Url: https://codereview.chromium.org/2178923002
4 files changed