rtpproto: Allow specifying a separate rtcp port in ff_rtp_set_remote_url

A separate rtcp port can already be set when opening the rtp
protocol normally, but when doing port setup as in RTSP (where
we first need to open the local ports and pass them to the peer,
and only then receive the remote peer port numbers), we didn't
check the same url parameter as in the normal open routine.

Signed-off-by: Martin Storsjö <martin@martin.st>