fix nav_menu

in commit 1d55f6089f4640ddcadd3f24f42482665645d36a, it remove the filter
to avoid an exception when adding custom site menu without submenu. But
it result in showing a raw menu title when the menu has submenu and user
has no permission to all the submenu items.
Merge pull request #187 from cgcgbcbc/fix/nav_menu

fix nav_menu