fix fuzzer crash

Fail if data is
inconsistent.

R=kjlubick@google.com
BUG=skia:

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=4923

Change-Id: Ie3f65c80c0d59968b1e3fc8bc91839bedb2e3da3
Reviewed-on: https://skia-review.googlesource.com/4923
Commit-Queue: Cary Clark <caryclark@google.com>
Reviewed-by: Kevin Lubick <kjlubick@google.com>