limit number of searched roots

Extreme numbers can generate more than
three found cubic roots when the roots
are found through a binary search.

Fail in this case.

TBR=reed@google.com
BUG=630649
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2176733002

Review-Url: https://codereview.chromium.org/2176733002
2 files changed