Fix for crash in Freestyle with sketchy chaining and Bezier Curve geometry modifier.

When the sketchy chaining is used, stroke geometry may contain a 180-degree U-turn.
If the 'error' parameter of the Bezier Curve geometry modifier is small (e.g., 10),
Bezier curve fitting will recursively split the original stroke into two pieces.
This splitting may take place at a U-turn point, causing a numerical singularity issue
that leads to a crash.

Problem report by edna in the BA Freestyle thread, with an example .blend to reproduce
the problem.  Thanks a lot!

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@57068 954f8c5b-7b00-dc11-b283-0030488c597c