Keep the cell_list in a consistent state while iterating over it, since

Keep the cell_list in a consistent state while iterating over it, since

Sat Mar 13 00:07:51 2004  Matthias Clasen  <maclas@gmx.de>

	* gtk/gtktreeviewcolumn.c (gtk_tree_view_column_cell_layout_clear):
	Keep the cell_list in a consistent state while iterating over it,
	since clear_attributes() also iterates over it.  (another instance
	of #136585, Morten Welinder)