check the return value of _gtk_tree_view_find_node and return if it's

check the return value of _gtk_tree_view_find_node and return if it's

Mon Apr  8 20:28:54 2002  Kristian Rietveld  <kris@gtk.org>

        * gtk/gtktreeselection.c (gtk_tree_selection_select_path),
        (gtk_tree_selection_unselect_path),
        (gtk_tree_selection_path_is_selected): check the return value of
        _gtk_tree_view_find_node and return if it's TRUE. This makes those
        functions work somewhat saner on non-expanded trees.