lavf: fix av_find_best_stream when providing a wanted stream.

In the main loop, stream_number is incremented after checking the stream type,
so the search usually will not find the wanted stream.

This patch eliminates the useless stream_number variable and introduces a new
one, called real_stream_index to store the real stream index of the current
stream, no matter if we are looping through all the streams or only the streams
of a program.
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
Signed-off-by: Anton Khirnov <anton@khirnov.net>

libavformat/utils.c

@@ -2462,7 +2462,7 @@ int av_find_best_stream(AVFormatContext *ic,
                         AVCodec **decoder_ret,
                         int flags)
 {
-    int i, nb_streams = ic->nb_streams, stream_number = 0;
+    int i, nb_streams = ic->nb_streams;
     int ret = AVERROR_STREAM_NOT_FOUND, best_count = -1;
     unsigned *program = NULL;
     AVCodec *decoder = NULL, *best_decoder = NULL;
@@ -2475,11 +2475,12 @@ int av_find_best_stream(AVFormatContext *ic,
         }
     }
     for (i = 0; i < nb_streams; i++) {
-        AVStream *st = ic->streams[program ? program[i] : i];
+        int real_stream_index = program ? program[i] : i;
+        AVStream *st = ic->streams[real_stream_index];
         AVCodecContext *avctx = st->codec;
         if (avctx->codec_type != type)
             continue;
-        if (wanted_stream_nb >= 0 && stream_number++ != wanted_stream_nb)
+        if (wanted_stream_nb >= 0 && real_stream_index != wanted_stream_nb)
             continue;
         if (st->disposition & (AV_DISPOSITION_HEARING_IMPAIRED|AV_DISPOSITION_VISUAL_IMPAIRED))
             continue;
@@ -2494,7 +2495,7 @@ int av_find_best_stream(AVFormatContext *ic,
         if (best_count >= st->codec_info_nb_frames)
             continue;
         best_count = st->codec_info_nb_frames;
-        ret = program ? program[i] : i;
+        ret = real_stream_index;
         best_decoder = decoder;
         if (program && i == nb_streams - 1 && ret < 0) {
             program = NULL;