allow conic chop to fail

Fuzzy values may cause the conic chop to fail.

Check to see if the values are all finite, and
require the caller to do the same.

R=reed@google.com
BUG=650178
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2368993002

Review-Url: https://codereview.chromium.org/2368993002
9 files changed