bug fix for extractor xiami.py

bug fix for extractor xiami.py

Before applying this patch, when downloading resources from xiami.com, it crashes with these:
Traceback (most recent call last):
  File "/home/phi/.local/bin/youtube-dl", line 11, in <module>
    sys.exit(main())
  File "/home/phi/.local/lib/python3.5/site-packages/youtube_dl/__init__.py", line 433, in main
    _real_main(argv)
  File "/home/phi/.local/lib/python3.5/site-packages/youtube_dl/__init__.py", line 423, in _real_main
    retcode = ydl.download(all_urls)
  File "/home/phi/.local/lib/python3.5/site-packages/youtube_dl/YoutubeDL.py", line 1786, in download
    url, force_generic_extractor=self.params.get('force_generic_extractor', False))
  File "/home/phi/.local/lib/python3.5/site-packages/youtube_dl/YoutubeDL.py", line 691, in extract_info
    ie_result = ie.extract(url)
  File "/home/phi/.local/lib/python3.5/site-packages/youtube_dl/extractor/common.py", line 347, in extract
    return self._real_extract(url)
  File "/home/phi/.local/lib/python3.5/site-packages/youtube_dl/extractor/xiami.py", line 116, in _real_extract
    return self._extract_tracks(self._match_id(url))[0]
  File "/home/phi/.local/lib/python3.5/site-packages/youtube_dl/extractor/xiami.py", line 43, in _extract_tracks
    '%s/%s%s' % (self._API_BASE_URL, item_id, '/type/%s' % typ if typ else ''), item_id)
  File "/home/phi/.local/lib/python3.5/site-packages/youtube_dl/extractor/common.py", line 562, in _download_json
    json_string, video_id, transform_source=transform_source, fatal=fatal)
  File "/home/phi/.local/lib/python3.5/site-packages/youtube_dl/extractor/common.py", line 568, in _parse_json
    return json.loads(json_string)
  File "/usr/lib/python3.5/json/__init__.py", line 312, in loads
    s.__class__.__name__))
TypeError: the JSON object must be str, not 'NoneType'

This patch solves exactly this problem.