client: set the watch to flushing before going to NULL
client: set the watch to flushing before going to NULL

First set the watch to flushing so that we unblock any current and
future attempt to send data on the watch, Then set the pipeline to
NULL.

Fixes https://bugzilla.gnome.org/show_bug.cgi?id=728153

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