rtsp-media: don't collect media stats when going to NULL
rtsp-media: don't collect media stats when going to NULL

Fixes https://bugzilla.gnome.org/show_bug.cgi?id=678015

surround-io/gst-rtsp-server,barisdemiray/gst-rtsp-server,thaytan/gst-rtsp-server,hsteinhaus/gst-rtsp-server,Lachann/gst-rtsp-server,thaytan/gst-rtsp-server,surround-io/gst-rtsp-server,hsteinhaus/gst-rtsp-server,barisdemiray/gst-rtsp-server,surround-io/gst-rtsp-server,Lachann/gst-rtsp-server,Lachann/gst-rtsp-server,barisdemiray/gst-rtsp-server,thaytan/gst-rtsp-server,hsteinhaus/gst-rtsp-server