Check the column title is not empty before using it

Check the column title is not empty before using it

Fixes crash with terminals that don't fit all titles.

Traceback (most recent call last):
  File "./iotop.py", line 12, in <module>
    main()
  File "./iotop/ui.py", line 669, in main
    main_loop()
  File "./iotop/ui.py", line 659, in <lambda>
    main_loop = lambda: run_iotop(options)
  File "./iotop/ui.py", line 554, in run_iotop
    return curses.wrapper(run_iotop_window, options)
  File "/usr/lib/python3.5/curses/__init__.py", line 94, in wrapper
    return func(stdscr, *args, **kwds)
  File "./iotop/ui.py", line 546, in run_iotop_window
    ui.run()
  File "./iotop/ui.py", line 176, in run
    self.process_list.duration)
  File "./iotop/ui.py", line 515, in refresh_display
    self.win.addstr(len_summary, pos, title, attr)
_curses.error: addwstr() returned ERR