wibox_set_border_color() and luaA_wibox_set_border_width() didn't check if a
wibox really had an associated X11 window before changing this window which
resulted in (silent) errors.
Signed-off-by: Uli Schlachter <psychon@znc.in>
Signed-off-by: Julien Danjou <julien@danjou.info>