Fix prescontrol crash at start-up

$ ./prescontrol/prescontrol file.pdf
Traceback (most recent call last):
  File "./prescontrol/prescontrol", line 185, in <module>
    curses.wrapper(application, args)
  File "/usr/lib64/python3.2/curses/wrapper.py", line 43, in wrapper
    return func(stdscr, *args, **kwds)
  File "./prescontrol/prescontrol", line 167, in application
    p.launch_preview_range(-args.left, args.right)
AttributeError: 'Namespace' object has no attribute 'left'