compositor: don't crash if destroying a compositor without a backend

Calling weston_compositor_destroy() on a pointer returned by
weston_compositor_create() should be always valid, even if the
compositor does not have yet a backend.

Signed-off-by: Giulio Camuffo <giuliocamuffo@gmail.com>
Reviewed-by: Derek Foreman <derekf@osg.samsung.com>

src/compositor.c

@@ -4767,7 +4767,8 @@ weston_compositor_destroy(struct weston_compositor *compositor)
 
 	weston_compositor_xkb_destroy(compositor);
 
-	compositor->backend->destroy(compositor);
+	if (compositor->backend)
+		compositor->backend->destroy(compositor);
 	free(compositor);
 }