don't crash if GVertex has no mesh_vertices

Mesh/meshGFaceOptimize.cpp

@@ -128,8 +128,10 @@ void buildMeshGenerationDataStructures(GFace *gf,
     std::list<GVertex*> emb_vertx = gf->embeddedVertices();
     std::list<GVertex*>::iterator itvx = emb_vertx.begin();
     while(itvx != emb_vertx.end()){
+      if((*itvx)->mesh_vertices.size()){
         MVertex *v = *((*itvx)->mesh_vertices.begin());
         vSizesMap[v] = std::min(vSizesMap[v], (*itvx)->prescribedMeshSizeAtVertex());
+      }
       ++itvx;
     }
   }