Linux: get_unrar() can return None

[Gfy]

Traceback (most recent call last):
  File "pyrescene.py", line 1678, in <module>
    sys.exit(main())
  File "pyrescene.py", line 1578, in main
    options, mthread)
  File "pyrescene.py", line 998, in generate_srr
    if options.vobsub_srr and not unrar_is_available():
  File "pyrescene.py", line 95, in unrar_is_available
    return os.path.isfile(os.path.abspath(get_unrar()))
  File "/usr/lib/python3.4/posixpath.py", line 357, in abspath
    if not isabs(path):
  File "/usr/lib/python3.4/posixpath.py", line 63, in isabs
    return s.startswith(sep)
AttributeError: 'NoneType' object has no attribute 'startswith'