Fix loopIndex in chopCubicAtLoopIntersection

A recent change broke the case where the entire bezier is inside the
loop segment.

Bug: skia:4410
Change-Id: Ib534d459eaa4461d6760e894a8826e3019584ee8
Reviewed-on: https://skia-review.googlesource.com/19333
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Chris Dalton <csmartdalton@google.com>
1 file changed