client: call weak-unref on client->sessions from finalize
client: call weak-unref on client->sessions from finalize

Fixes bug #596305

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