gridview: Allow starting rubberband in empty space below last row

Rubberband does not work when initiated past the last row
(warning is printed "Could not start rubberbanding: No item).

Clamp y at the max height of the widgets in the gridview

Fixes: #3462

gtk/gtkgridview.c

@@ -441,6 +441,7 @@ gtk_grid_view_get_position_from_allocation (GtkListBase           *base,
     return FALSE;
 
   n_items = gtk_list_base_get_n_items (base);
+  along = CLAMP (along, 0, gtk_grid_view_compute_total_height (self) - 1);
   if (!gtk_grid_view_get_cell_at_y (self,
                                     along,
                                     &pos,