[postprocessor/ffmpeg] Don't let ffmpeg read from stdin (fixes #4945)

If you run 'while read aurl ; do youtube-dl --extract-audio "${aurl}"; done < path_to_batch_file'  (batch_file contains one url per line) each call to youtube-dl consumed some characters and 'read' would assing to 'aurl' a non valid url, something like 'tube.com/watch?v=<id>'.