--write-sub causes AttributeError: 'tuple' object has no attribute 'keys'

[ivan]

$ git rev-parse HEAD
f10b2a9c14db686e7f9b7d050f41b26d5cc35e01

$ python -m youtube_dl --write-sub "https://www.youtube.com/watch?v=sZtfnC2CyU0"
[youtube] Setting language
[youtube] sZtfnC2CyU0: Downloading video webpage
[youtube] sZtfnC2CyU0: Downloading video info webpage
[youtube] sZtfnC2CyU0: Extracting video information
[youtube] sZtfnC2CyU0: Checking available subtitles
Traceback (most recent call last):
  File "/usr/lib/python2.7/runpy.py", line 162, in _run_module_as_main
    "__main__", fname, loader, pkg_name)
  File "/usr/lib/python2.7/runpy.py", line 72, in _run_code
    exec code in run_globals
  File "/home/at/trash/youtube-dl/youtube_dl/__main__.py", line 17, in <module>
    youtube_dl.main()
  File "youtube_dl/__init__.py", line 540, in main
    _real_main()
  File "youtube_dl/__init__.py", line 524, in _real_main
    retcode = fd.download(all_urls)
  File "youtube_dl/FileDownloader.py", line 547, in download
    videos = ie.extract(url)
  File "youtube_dl/InfoExtractors.py", line 96, in extract
    return self._real_extract(url)
  File "youtube_dl/InfoExtractors.py", line 531, in _real_extract
    video_subtitles = self._extract_subtitle(video_id)
  File "youtube_dl/InfoExtractors.py", line 292, in _extract_subtitle
    sub_lang = list(sub_lang_list.keys())[0]
AttributeError: 'tuple' object has no attribute 'keys'

[jaimeMF]

The problem is that youtube-dl doesn't find subtitles and _get_available_subtitles returns a tuple with the warning message

[FiloSottile]

Fixed by @jaimeMF in #764