Fix av_find_best_stream when using a program

The current implementation has a bug, it is returning the stream index
in the found program, and not the stream index in the list of all
streams. The attached patch fixes this issue.

Signed-off-by: Mans Rullgard <mans@mansr.com>

libavformat/utils.c

@@ -2529,7 +2529,7 @@ int av_find_best_stream(AVFormatContext *ic,
         if (best_count >= st->codec_info_nb_frames)
             continue;
         best_count = st->codec_info_nb_frames;
-        ret = i;
+        ret = program ? program[i] : i;
         best_decoder = decoder;
         if (program && i == nb_streams - 1 && ret < 0) {
             program = NULL;