fullscreen-shell: Avoid NULL pointer dereference

When there are no outputs present, an output pointer
can be NULL. Dereferencing such pointer will result
in a crash.

Signed-off-by: Armin Krezović <krezovic.armin@gmail.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>

fullscreen-shell/fullscreen-shell.c

@@ -288,7 +288,7 @@ fs_output_for_output(struct weston_output *output)
 static void
 restore_output_mode(struct weston_output *output)
 {
-	if (output->original_mode)
+	if (output && output->original_mode)
 		weston_output_mode_switch_to_native(output);
 }