compositor-fbdev: fix VT switching crash

This bug was introduced in 954f183e2f.

The session_notify() data was accidentally cast to fbdev_backend while
it is weston_compositor. This was possibly due to the code before the
mentioned commit casting data directly to fbdev_compositor without going
through the intended type first, which worked only because
weston_compositor was the first member in struct fbdev_compositor.

Fix the casts to be the right way around.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=91654
Cc: nerdopolis1@verizon.net
Cc: Giulio Camuffo <giuliocamuffo@gmail.com>
Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Derek Foreman <derekf@osg.samsung.com>

src/compositor-fbdev.c

@@ -756,8 +756,8 @@ fbdev_backend_destroy(struct weston_compositor *base)
 static void
 session_notify(struct wl_listener *listener, void *data)
 {
-	struct fbdev_backend *backend = data;
-	struct weston_compositor *compositor = backend->compositor;
+	struct weston_compositor *compositor = data;
+	struct fbdev_backend *backend = to_fbdev_backend(compositor);
 	struct weston_output *output;
 
 	if (compositor->session_active) {