fix cubic linear test

Check to see if the line between end points is
degenerate before measuring control points.

Also, add test case for a bug to see if it
shows up on any platform.

TBR=reed@google.com
BUG=skia:5169, skia:5240
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2375053002

Review-Url: https://codereview.chromium.org/2375053002
5 files changed