SEGFAULT Fix

- when deleting a Scene_edit_polyhedron_item WHILE manipulating a frame, a segfault
occured because a bad manipulated frame was sent to the viewer. Calling setVisible(false)
 in the destructor of the item calls setManipulatedFrame(NULL) and fixex the problem.

Polyhedron/demo/Polyhedron/Scene_edit_polyhedron_item.cpp

@@ -110,6 +110,7 @@ Scene_edit_polyhedron_item::Scene_edit_polyhedron_item
 
 Scene_edit_polyhedron_item::~Scene_edit_polyhedron_item()
 {
+    setVisible(false);
   while(is_there_any_ctrl_vertices_group())
   {
     delete_ctrl_vertices_group(false);