Fallback to moveTo when unable to find the first tangent in cubicTo

When calling cubicTo(a, b, c) and if the distance between fPrevPt and a
is too small, b is used instead of a to calculate the first tangent,
even if the distance between fPrevPt and b is too small.

In debug mode, this is causing an assertion to fail in
SkPathStroker::preJoinTo() and, in Release, the use of an
unitialized value.

The first patch set is adding a failing test.
The second one add the fix to SkPathStroker::cubicTo()

R=bsalomon@chromium.org, junov@chromium.org, reed@google.com, caryclark@google.com, bsalomon@google.com

Author: piotaixr@chromium.org

Review URL: https://codereview.chromium.org/460813002

(cherry picked from commit fac4e0e83666ab59373169d6c157d3654cb479a3)

Review URL: https://codereview.chromium.org/546823002
2 files changed