rtsp-client: Protect saved clients watch with a mutex
rtsp-client: Protect saved clients watch with a mutex

Fixes a crash when close() is called while merging clients
in handle_tunnel(). In that case close() would destroy the
watch while it is still being used in handle_tunnel().

https://bugzilla.gnome.org/show_bug.cgi?id=735570

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