Fix possible crash when decoding mpeg streams.

This reverts 2cf8355f,
fixes ticket 329.

ffmpeg.c

@@ -335,6 +335,7 @@ typedef struct InputFile {
     int eof_reached;      /* true if eof reached */
     int ist_index;        /* index of first stream in ist_table */
     int buffer_size;      /* current total buffer size */
+    int nb_streams;
     int64_t ts_offset;
 } InputFile;
 
@@ -2063,7 +2064,7 @@ 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;
@@ -2769,7 +2770,7 @@ 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];
@@ -3490,6 +3491,7 @@ 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;
 
     top_field_first = -1;
     frame_rate    = (AVRational){0, 0};