rtpsession: don't unref buffer twice

Cleaning the packet info will already unref the buffer.

Fixes https://bugzilla.gnome.org/show_bug.cgi?id=715078

gst/rtpmanager/rtpsession.c

@@ -1833,7 +1833,6 @@ rtp_session_process_rtp (RTPSession * sess, GstBuffer * buffer,
 collision:
   {
     RTP_SESSION_UNLOCK (sess);
-    gst_buffer_unref (buffer);
     clean_packet_info (&pinfo);
     GST_DEBUG ("ignoring packet because its collisioning");
     return GST_FLOW_OK;