rtcpbuffer: fix sanity check for gst_rtcp_packet_xr_get_prt_info()
It's not quite clear to me how the check matches the spec though - I'm guesing it was supposed to use
*start_seq, but I think it would also have to take into account thinning? And is the min size calculation correct? (Isn't it more then 8 bits per seqnum?)
In general it also looks to me at first glance like sizes aren't properly checked in many places in the various new XR parsing functions before advancing pointers and such, but perhaps I misread that.