break out of cubic stroker loop on degenerate case

The looper can generate more than one quad, but if any one is degenerate,
give up, but not before generating the state for the line join to
produce the correct end.

Before, the early return allowed the inside path to contain multiple
movetos that caused reversePath to assert.

R=reed@google.com

Review URL: https://codereview.chromium.org/948043002
2 files changed