prevent `--` from being added to the list of choices

[tinruufu]

i have no idea how this happened, but under homebrew, before installing unrar (which may or may not be relevant):

Traceback (most recent call last):
  File "/usr/local/lib/python3.5/site-packages/flask/app.py", line 1988, in wsgi_app
    response = self.full_dispatch_request()
  File "/usr/local/lib/python3.5/site-packages/flask/app.py", line 1641, in full_dispatch_request
    rv = self.handle_user_exception(e)
  File "/usr/local/lib/python3.5/site-packages/flask/app.py", line 1544, in handle_user_exception
    reraise(exc_type, exc_value, tb)
  File "/usr/local/lib/python3.5/site-packages/flask/_compat.py", line 33, in reraise
    raise value
  File "/usr/local/lib/python3.5/site-packages/flask/app.py", line 1639, in full_dispatch_request
    rv = self.dispatch_request()
  File "/usr/local/lib/python3.5/site-packages/flask/app.py", line 1625, in dispatch_request
    return self.view_functions[rule.endpoint](**req.view_args)
  File "/usr/local/lib/python3.5/site-packages/friction/__init__.py", line 53, in items
    doujin = library.choice(request.args.get('f', '').strip())
  File "/usr/local/lib/python3.5/site-packages/friction/library.py", line 154, in choice
    return self.doujin_for(choice(choices_list)[0])
  File "/usr/local/lib/python3.5/site-packages/friction/library.py", line 125, in doujin_for
    ARCHIVE_HANDLERS[mime](full_path, target)
  File "/usr/local/lib/python3.5/site-packages/friction/library.py", line 26, in extract_rar
    RarFile(source).extractall(dest)
  File "/usr/local/lib/python3.5/site-packages/rarfile.py", line 679, in extractall
    self._extract(fnlist, path, pwd)
  File "/usr/local/lib/python3.5/site-packages/rarfile.py", line 1238, in _extract
    check_returncode(p, output)
  File "/usr/local/lib/python3.5/site-packages/rarfile.py", line 1961, in check_returncode
    raise exc(msg)
rarfile.RarUnknownError: Unknown exit code [1]: b"bsdtar: Error opening archive: Failed to open '--'\n"