Fix bug in tesselation

When we hit a self-intersection, we need to be careful that we don't
lose the stop vertex.

Fixes: lp:1833819
* https://bugs.launchpad.net/kicad/+bug/1833819

(cherry picked from commit 6d60b98dc8)

include/geometry/polygon_triangulation.h

@@ -1,7 +1,7 @@
 /*
  * This program source code file is part of KiCad, a free EDA CAD application.
  *
- * Modifications Copyright (C) 2018 KiCad Developers
+ * Modifications Copyright (C) 2018-2019 KiCad Developers
  *
  * This program is free software; you can redistribute it and/or
  * modify it under the terms of the GNU General Public License
@@ -420,6 +420,7 @@ private:
                 aPoint->remove();
 
                 aPoint = nextNext;
+                stop = nextNext;
 
                 continue;
             }