fix pathops fuzz

Extreme cubics may split so that one half is a point.
Discard this rather than generating a degenerate line.

TBR=reed@google.com
BUG=640025
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2276503002

Review-Url: https://codereview.chromium.org/2276503002
4 files changed