Fix for rare crash in Poly::addEdge().

Don't add an edge if the bottom vertex was already added, or
if an island vertex has a left poly but no right poly.

(Sorry for the lack of test, but the only reduction I could create was still a huge path and only crashes in Chrome.)

BUG=617907
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2043873005

Review-Url: https://codereview.chromium.org/2043873005
1 file changed