Revert "ffmpeg: get rid of useless AVInputStream.nb_streams."

This reverts commit 2cf8355f98.
AVInputStream.nb_streams tracks number of streams found at the
beginning, new streams may appear that ffmpeg doesn't know about. Fixes
crash in this case.

ffmpeg.c

@@ -322,6 +322,7 @@ typedef struct InputFile {
     int ist_index;        /* index of first stream in ist_table */
     int buffer_size;      /* current total buffer size */
     int64_t ts_offset;
+    int nb_streams;       /* nb streams we are aware of */
 } InputFile;
 
 static InputStream *input_streams = NULL;
@@ -1968,7 +1969,7 @@ static int transcode(AVFormatContext **output_files,
         int si = stream_maps[i].stream_index;
 
         if (fi < 0 || fi > nb_input_files - 1 ||
-            si < 0 || si > input_files[fi].ctx->nb_streams - 1) {
+            si < 0 || si > input_files[fi].nb_streams - 1) {
             fprintf(stderr,"Could not find input stream #%d.%d\n", fi, si);
             ret = AVERROR(EINVAL);
             goto fail;
@@ -1976,7 +1977,7 @@ static int transcode(AVFormatContext **output_files,
         fi = stream_maps[i].sync_file_index;
         si = stream_maps[i].sync_stream_index;
         if (fi < 0 || fi > nb_input_files - 1 ||
-            si < 0 || si > input_files[fi].ctx->nb_streams - 1) {
+            si < 0 || si > input_files[fi].nb_streams - 1) {
             fprintf(stderr,"Could not find sync stream #%d.%d\n", fi, si);
             ret = AVERROR(EINVAL);
             goto fail;
@@ -2600,7 +2601,7 @@ static int transcode(AVFormatContext **output_files,
         }
         /* the following test is needed in case new streams appear
            dynamically in stream : we ignore them */
-        if (pkt.stream_index >= input_files[file_index].ctx->nb_streams)
+        if (pkt.stream_index >= input_files[file_index].nb_streams)
             goto discard_packet;
         ist_index = input_files[file_index].ist_index + pkt.stream_index;
         ist = &input_streams[ist_index];
@@ -3326,6 +3327,7 @@ static int opt_input_file(const char *opt, const char *filename)
     input_files[nb_input_files - 1].ctx        = ic;
     input_files[nb_input_files - 1].ist_index  = nb_input_streams - ic->nb_streams;
     input_files[nb_input_files - 1].ts_offset  = input_ts_offset - (copy_ts ? 0 : timestamp);
+    input_files[nb_input_files - 1].nb_streams = ic->nb_streams;
 
     frame_rate    = (AVRational){0, 0};
     frame_pix_fmt = PIX_FMT_NONE;