Send BYE packets immediatly for small sessions

When the number of participants is less than 50, the RFC allows for sending the
BYE packet immediatly instead of using the regular BYE timeout.
Fixes #567828.

gst/rtpmanager/rtpstats.c

@@ -145,6 +145,10 @@ rtp_stats_calculate_bye_interval (RTPSessionStats * stats)
   gdouble interval;
   gdouble rtcp_min_time;
 
+  /* no interval when we have less than 50 members */
+  if (stats->active_sources < 50)
+    return 0;
+  
   rtcp_min_time = (stats->min_interval) / 2.0;
 
   /* Dedicate a fraction of the RTCP bandwidth to senders unless