rtsp/rdt: Assign the RTSPStream index to AVStream->id

This is used for mapping AVStreams back to their corresponding
RTSPStream. Since d9c0510, the RTSPStream pointer isn't stored in
AVStream->priv_data any longer, breaking this mapping from AVStreams
to RTSPStreams.

Also, we don't need to clear the priv_data in rdt cleanup any longer,
since it isn't set to duplicate pointers.

Signed-off-by: Ronald S. Bultje <rsbultje@gmail.com>
(cherry picked from commit b2dd842d21)

libavformat/rtspdec.c

@@ -283,7 +283,7 @@ retry:
             for (i = 0; i < rt->nb_rtsp_streams; i++) {
                 rule_nr = 0;
                 for (r = 0; r < s->nb_streams; r++) {
-                    if (s->streams[r]->priv_data == rt->rtsp_streams[i]) {
+                    if (s->streams[r]->id == i) {
                         if (s->streams[r]->discard != AVDISCARD_ALL) {
                             if (!first)
                                 av_strlcat(rt->last_subscription, ",",