fix another pathops fuzz bug

Tunnel out the error case when the numerics
create uncalculable angles.

R=mmoroz@chromium.org
TBR=reed@google.com
BUG=624351
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2103863005

Review-Url: https://codereview.chromium.org/2103863005
3 files changed