gridview: Add an assertion

The want to use the footer tile at the end
to fill leftover space at the bottome right.

So lets assert that we actually dealing with
a footer tile, just in case something changes
in the future that might have us end up with
some other kind of tile.

gtk/gtkgridview.c

@@ -885,6 +885,7 @@ gtk_grid_view_size_allocate (GtkWidget *widget,
     {
       GtkListTile *filler;
       tile = gtk_list_item_manager_get_last (self->item_manager);
+      g_assert (gtk_list_tile_is_footer (tile));
       filler = gtk_list_tile_append_filler (self->item_manager, tile);
       gtk_list_tile_set_area_position (self->item_manager,
                                        filler,