* src/filebuffer.cc (FileBuffer::on_mark_deleted): Use swap() to
reset Glib::RefPtr<> because neither clear() nor reset() can be
relied upon to be there.
* src/filetree.cc (FileTree::save_file_at_iter): ditto,
(FileTree::find_matches_at_path_iter): ditto,
(FileTree::on_selection_changed): ditto.