ffmpeg: let AVFilterGraph parse the filter_threads option

This way the CLI accepts for "filter_threads" the same values as for the
libavcodec specific option "threads".

Fixes FATE with THREADS=auto which was broken in bdc1bdf3f5.

Signed-off-by: James Almer <jamrial@gmail.com>

fftools/ffmpeg.c

@@ -653,6 +653,7 @@ static void ffmpeg_cleanup(int ret)
                    av_err2str(AVERROR(errno)));
     }
     av_freep(&vstats_filename);
+    av_freep(&filter_nbthreads);
 
     av_freep(&input_streams);
     av_freep(&input_files);