BP Nav Menu: If menu URL doesn't exist, mark menu item as invalid.

Previously, when a BuddyPress component is deactivated but a menu item
for this component still exists, the generated menu would create a warning.

This commit addresses this.

Fixes #5545 (trunk)

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8308 cdf35c40-ae34-48e0-9cc9-0c9da1808c22