fix ExtrusionLoop::split_at() and improved ExtrusionLoop::split_at_ve…

…rtex()

src/libslic3r/ExtrusionEntity.cpp

@@ -159,7 +159,7 @@ bool ExtrusionLoop::split_at_vertex(const Point &point, const double scaled_epsi
  p2.append(std::move(p1));
  path->polyline.swap(p2); // swap points & fitting result
  }
- } else {
+ } else if (idx > 0 && idx < path->size() - 1) {
  // new paths list starts with the second half of current path
  ExtrusionPaths new_paths;
  PolylineOrArc p1, p2;
@@ -236,8 +236,8 @@ void ExtrusionLoop::split_at(const Point &point, bool prefer_non_overhang, const
  const Point *p1 = this->paths[path_idx].polyline.get_points().data() + segment_idx;
  const Point *p2 = p1;
  ++ p2;
- double d2_1 = (point - *p1).cast<double>().squaredNorm();
- double d2_2 = (point - *p2).cast<double>().squaredNorm();
+ double d2_1 = (p - *p1).cast<double>().squaredNorm();
+ double d2_2 = (p - *p2).cast<double>().squaredNorm();
  const double thr2 = scaled_epsilon * scaled_epsilon;
  if (d2_1 < d2_2) {
  if (d2_1 < thr2)