If someone is running youtube-dl on a server to deliver files, the user could input 'file:///some/important/file' and youtube-dl would save that file as a video giving access to sensitive information to the user.
'file:' urls can be filtered, but the user can use an URL to a crafted m3u8 manifest like:
#EXTM3U
#EXT-X-MEDIA-SEQUENCE:0
#EXTINF:10.0
file:///etc/passwd
#EXT-X-ENDLIST
With this patch 'file:' URLs raise URLError like for unknown protocols.
In RFC6381, there's no rule stating that the first part of codecs should
be video and the second part should be audio, while it seems the case
for data reported by YouTube.
In some dailymotion videos (like http://www.dailymotion.com/video/x3k0dtv from #8156) the segments URLs don't have the 'Content-Length' header and HttpFD sets the 'totat_bytes' field to None, so we also use '0' in that case (since we do different math operations with it).