menubar: Fix refresh without show

When menubar.refresh() is called, it tries to update the menubar widget.
The call chain looks like this: menulist_update -> common.list_update ->
get_current_page. get_current_page then tries to query information about
the size of the menubar.

Since there is not much point in this, just skip the whole callback in
this case.

Side note: What is the point of menubar.get()? It seems quite useless to
me.

Signed-off-by: Uli Schlachter <psychon@znc.in>

lib/menubar/init.lua

@@ -350,7 +350,9 @@ end
 function menubar.refresh(scr)
     menubar.menu_gen.generate(function(entries)
         menubar.menu_entries = entries
+        if instance then
             menulist_update(nil, scr)
+        end
     end)
 end