menu: Fix regression

There was already a bug, as self.active_child.visible
was used instead of self.active_child.wibox.visible

This caused some confusion that this attribute was a widget.
It wasn't.

lib/awful/menu.lua

@@ -249,7 +249,7 @@ function menu:exec(num, opts)
             self.active_child:hide()
         end
         self.active_child = self.child[num]
-        if not self.active_child:get_visible() then
+        if not self.active_child.wibox.visible then
             self.active_child:show()
         end
     elseif type(cmd) == "string" then