[downloader/fragment] Don't fail if the 'Content-Length' header is missing

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).