Fix ibus-setup SEGV in enginetreeview.py

When an engine is added on ibus-setup, enginetreeview.append_engine()
calls self.__model.append(None) and it causes engine == None in
__icon_cell_data_cb() and __name_cell_data_cb() in enginetreeview
with python3-gobject 3.13.x which version has implemented several
deprecated warnings in gi/overrides/Gtk.py .
I think this problem also could happen in the old versions if
the system would be slow.

BUG=rhbz#1048429
TEST=setup in Fedora 21

Review URL: https://codereview.appspot.com/104620043