xserver: check whether pointer exists when exiting

At initialization, if it fails in binding the socket or creating the
lock file then the pointer will be already freed and will result a
segfault when quiting the compositor.

Signed-off-by: Tiago Vignatti <tiago.vignatti@intel.com>

compositor/xserver-launcher.c

@@ -808,6 +808,9 @@ wlsc_xserver_destroy(struct wlsc_compositor *compositor)
 {
 	struct wlsc_xserver *wxs = compositor->wxs;
 
+	if (!wxs)
+		return;
+
 	if (wxs->loop)
 		wlsc_xserver_shutdown(wxs);