fftools/ffmpeg: discard packets for unused streams in demuxing thread

Avoids the pointless overhead of transferring them to the main thread.

fftools/ffmpeg.c

@@ -1961,9 +1961,6 @@ static int process_input(int file_index)
     ist->data_size += pkt->size;
     ist->nb_packets++;
 
-    if (ist->discard)
-        goto discard_packet;
-
     /* add the stream-global side data to the first packet */
     if (ist->nb_packets == 1) {
         for (i = 0; i < ist->st->nb_side_data; i++) {
@@ -2002,7 +1999,6 @@ static int process_input(int file_index)
 
     process_input_packet(ist, pkt, 0);
 
-discard_packet:
     av_packet_free(&pkt);
 
     return 0;

fftools/ffmpeg_demux.c

@@ -318,7 +318,8 @@ static void *input_thread(void *arg)
 
         /* the following test is needed in case new streams appear
            dynamically in stream : we ignore them */
-        if (pkt->stream_index >= f->nb_streams) {
+        if (pkt->stream_index >= f->nb_streams ||
+            f->streams[pkt->stream_index]->discard) {
             report_new_stream(d, pkt);
             av_packet_unref(pkt);
             continue;