rtsp: Don't store RTSPStream in AVStream->priv_data

For mpegts in RTP, there isn't a direct mapping between RTSPStreams
and AVStreams, and the RTSPStream isn't ever stored in
AVStream->priv_data, which was earlier leaked. The fix for this
leak, in ea7f080749d68a431226ce196014da38761a0d82, lead to
double frees for other, normal RTP streams.

This patch avoids storing RTSPStreams in AVStream->priv_data, thus
avoiding the double free. The RTSPStreams are always available via
RTSPState->rtsp_streams anyway.

Tested with MS-RTSP, RealRTSP, DSS and mpegts/RTP.

Signed-off-by: Luca Barbato <lu_zero@gentoo.org>
(cherry picked from commit d9c0510e22821baa364306d867ffac45da0620c8)
