wibox: check if a wibox' window exists before modifying it

If you change the .bg of an existing wibox which isn't attached to any screen
yet, then there is no window for which we can update the XCB_CW_BACK_PIXEL.

Signed-off-by: Uli Schlachter <psychon@znc.in>
Signed-off-by: Julien Danjou <julien@danjou.info>

objects/wibox.c

@@ -1010,6 +1010,8 @@ luaA_wibox_set_bg(lua_State *L, wibox_t *wibox)
         uint32_t values[] = { wibox->ctx.bg.pixel };
 
         wibox->need_update = true;
+
+        if (wibox->window != XCB_NONE)
             xcb_change_window_attributes(globalconf.connection,
                                          wibox->window,
                                          mask,