Lab - Fixed loading of OpenMesh files without selection (#8832)

## Summary of Changes

A `Scene_polyhedron_selection_item` is created when loading an OpenMesh
file. When there was no selection inside the file, this selection item
was neither added to the scene nor deleted. Thus, it still received Qt
signals and caused crashes when the actual mesh was removed.

## Release Management

* Affected package(s): CGALlab

Lab/demo/Lab/Plugins/IO/OM_io_plugin.cpp

@@ -124,6 +124,8 @@ load(QFileInfo fileinfo, bool& ok, bool add_to_scene){
       res << item;
       if(!selection_item->isEmpty())
         res << selection_item;
+      else
+        delete selection_item;
       return res;
     }
   }