gtktreeviewaccessible: do not trigger an assertion

gtktreeviewaccessible: do not trigger an assertion

The following assertion was triggered in model_row_inserted() because
iterate_thru_children() modifies the parameter tree_model before passing it to
traverse_cells().

Gtk-CRITICAL **: gtk_tree_path_compare: assertion `b->depth > 0' failed

The stack with the bug was:

  #0  gtk_tree_path_compare at gtktreemodel.c
  #1  traverse_cells at gtktreeviewaccessible.c
  #2  model_row_inserted at gtktreeviewaccessible.c

This patch calls iterate_thru_children() with a copy of the path so that the
original is not modified.