Help static analysis with an assertion

It is hard for clang to see that layouts will
always be smaller than num_layouts, so just assert
that.

gdk/wayland/gdkkeys-wayland.c

@@ -412,6 +412,7 @@ update_direction (GdkWaylandKeymap *keymap)
        gint layouts, layout;
 
        layouts = xkb_keymap_num_layouts_for_key (keymap->xkb_keymap, key);
+       g_assert (layouts <= num_layouts);
        for (layout = 0; layout < layouts; layout++)
          {
            const xkb_keysym_t *syms;