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
b2dd842d21a0b441bb9f7092357f479beb6b6f69)