Use get_registered_nav_menus() instead of $_wp_registered_nav_menus

[nacin]

Follow up to #27735. Whenever we're only reading from this value, we should use the public API.

[jesin]

I could only find 4 uses of $_wp_registered_nav_menus in wp-includes/nav-menu.php.

1. register_nav_menus()
2. unregister_nav_menu()
3. get_registered_nav_menus()
4. has_nav_menu()

This variable is modified in 1 and 2. So the only valid change would be 4. I think this patch should do.

I couldn't find other occurrences of $_wp_registered_nav_menus.

$grep -rn "\$_wp_registered_nav_menus" --exclude-dir=.svn src/ | cut -d':' -f1-2

src/wp-includes/nav-menu.php:74
src/wp-includes/nav-menu.php:78
src/wp-includes/nav-menu.php:89
src/wp-includes/nav-menu.php:91
src/wp-includes/nav-menu.php:92
src/wp-includes/nav-menu.php:93
src/wp-includes/nav-menu.php:119
src/wp-includes/nav-menu.php:120
src/wp-includes/nav-menu.php:121
src/wp-includes/nav-menu.php:145
src/wp-includes/nav-menu.php:147

[voldemortensen]

My screen didn't refresh before I added the patch. If comment:1 is correct, my patch can be ignored.

[SergeyBiryukov]

In 29717:

Use get_registered_nav_menus() in has_nav_menu().

props voldemortensen.
fixes #29461.