Fix yet another convexicator issue

This CL fixes the case where a bad initial vector (i.e., nearly zero) managed to short circuit all of the convexicator's logic. The initial bad vector would become the last vector and then never get displaced.

The history of this is:

https://codereview.chromium.org/298973004/
Switched the convexicator to not advance the last vector when the cross product wasn't significant

https://codereview.chromium.org/573763002/
Fixed a bug (crbug.com/412640) wherein a zero area path was being incorrectly categorized as convex b.c. opposite but equal vectors were not signaling concavity.

BUG=433683

Review URL: https://codereview.chromium.org/727283003
2 files changed