Bug fix

Fix #2456

When two clusters share a subsegment, the subsegment is split at its
middle, instead of using the length imposed by the clusters. If the
clusters were partially refined before that subsegment is split, one
must reset the clusters, because the minimal length may have changed.

Mesh_2/include/CGAL/Mesh_2/Clusters.h

@@ -325,6 +325,16 @@ update_cluster(Cluster& c, iterator it, Vertex_handle va,
   c.vertices.erase(vb);
   c.vertices[vm] = reduction;
 
+  if(false == reduction) {
+    for(typename Cluster::Vertices_map::iterator
+          it = c.vertices.begin(),
+          end = c.vertices.end();
+        it != end; ++it)
+    {
+      it->second = false;
+    }
+  }
+
   if(vb==c.smallest_angle.first)
     c.smallest_angle.first = vm;
   if(vb==c.smallest_angle.second)