rtsp-client: Avoid reuse of channel numbers for interleaved

rtsp-client: Avoid reuse of channel numbers for interleaved

If a (strange) client would reuse interleaved channel numbers in
multiple SETUP requests, we should not accept them. The channel
numbers are used for looking up stream transports in the
priv->transports hash table, and transports disappear from the table
if channel numbers are reused.

RFC 7826 (RTSP 2.0), Section 18.54, clarifies that it is OK for the
server to change the channel numbers suggested by the client.

https://bugzilla.gnome.org/show_bug.cgi?id=796988