Customize: Selective refresh for nav menus can cause infinite reloads when it contains invalid menu items

[westonruter]

The way I've reproduced the problem, as suggested by @swissspidy:

1. Create 5 posts.
2. Create a new nav menu and assign to a location.
3. Assign 5 posts to new nav menu.
4. Trash the 5 posts so that all of the nav menu items become invalid.
5. Load site in the customizer and notice infinite reloads.

It appears to be a problem with handleUnplacedNavMenuInstances.

Reported at:
​https://wordpress.org/support/topic/customizer-preview-keeps-refreshing-infinitely-under-certain-circumstances/#post-8463765
​https://wordpress.slack.com/archives/core-customize/p1479733895000901

I can confirm that 38890.0.diff​ resolves the issue I was having. Thank you!

In 39333:

Customize: Prevent selective refresh from causing infinite fallback refreshes when nav menu contains invalid items.

Invalid nav menu items are exported as settings in the customize controls since it is_admin(), but any such nav_menu_item settings are excluded from the preview since it's the frontend. Selective refresh for nav menus needs to ignore the syncing of any such nav_menu_item settings that are _invalid since they are never rendered, and when a nav menu only contains invalid items, an infinite fallback refresh can ensue.

Fixes #38890.