p3d_process_n crashes if an object has an empty slice

[TacogentP3D]

The fix is to check if sect.len() != 0 in

p3d/src/lib.rs

Lines 133 to 141 in cfd70b0

	let sect = if let AlgoType::Grid2dV3a = algo {
	intersect_2(&mesh, z_sect, step * 0.01)
	} else {
	intersect(&mesh, z_sect)
	};
	let cntr = get_contour(sect);
	if cntr.len() > 0 {
	cntrs.push(cntr);
	}

[ghost]

check exists in find_top_std_2、find_top_std_3、find_top_std_4。