[downloader/hls] Don't let ffmpeg read from stdin (#8139)

If you run 'while read aurl ; do youtube-dl "${aurl}"; done < path_to_batch_file'  (batch_file contains one url per line that uses the hls downloader) each call to youtube-dl consumed some characters and 'read' would assing to 'aurl' a non valid url

(This is the same problem that was fixed for the ffmpeg postprocessors in cffcbc02de)

youtube_dl/downloader/hls.py

@@ -46,7 +46,7 @@ class HlsFD(FileDownloader):
 
         self._debug_cmd(args)
 
-        retval = subprocess.call(args)
+        retval = subprocess.call(args, stdin=subprocess.PIPE)
         if retval == 0:
             fsize = os.path.getsize(encodeFilename(tmpfilename))
             self.to_screen('\r[%s] %s bytes' % (args[0], fsize))