Undefined offset on pages without permission

[wpdavis]

On pages in wp-admin/network, I was getting this error if multisite was not enabled:

Notice: Undefined offset: 0 in wp-admin/includes/menu.php on line 152

Attached patch moves the call to add_menu_classes(); until after the check if the user can access the page.

Avoid undefined offset error by moving call to add_menu_classes() until after the permissions check

Saw this too, but didn't care so much :)

(In [16471]) Fix undefined offset warnings on pages the current user does not have perms for. Props wpdavis. fixes #15469