fix fuzzer

turn asserts into error returns to fix fuzz

R=kjlubick@google.com
BUG=skia:6162

Change-Id: I7508eb5cb41425dcfc2718aaa9cc1ee8aa00acc9
Reviewed-on: https://skia-review.googlesource.com/9118
Reviewed-by: Kevin Lubick <kjlubick@google.com>
Commit-Queue: Cary Clark <caryclark@google.com>
1 file changed