debig message for zero return value in q: segment

Signed-off-by: Panagiotis Cheilaris <philaris@cs.ntua.gr>

Segment_Delaunay_graph_Linf_2/include/CGAL/Segment_Delaunay_graph_Linf_2/Voronoi_vertex_sqrt_field_new_C2.h

@@ -2039,6 +2039,8 @@ private:
             }
           } // end of samey case
         }
+        CGAL_SDG_DEBUG(std::cout << "debug return ZERO in case"
+            << " of q: segment (last resort)" << std::endl;);
         return ZERO;
       }