Nav Menu: Bug while assigning a new menu to an existing menu location

[ocean90]

Steps to reproduce:

• Theme with menu support
• get_theme_mod( 'nav_menu_locations' ) needs to be empty or shouldn't include the menu location for the next steps
• Go to Manage Locations and select Use new menu
• Create the new menu and save it

Expected: The menu is assigned to the location.
Current: It's not assigned.

get_theme_mod( 'nav_menu_locations' ) doesn't return all registered menu locations means the isset() check fails.

[ocean90]

25064.patch​ uses get_registered_nav_menus() for the isset() check.

[nofearinc]

small optimization added in wp-includes/nav-menu.php

[nacin]

In 25195:

Nav menus: Allow assigning a new menu to an existing location when no theme setting is currently saved.

props ocean90.
see #25064, for trunk.

[nacin]

In 25196:

Use get_nav_menu_locations() in wp_delete_nav_menu().

props nofearinc.
see #25064.

[nacin]

I suggest we only merge [25195] to 3.6.1.

[nacin]

In 25232:

Nav menus: Allow assigning a new menu to an existing location when no theme setting is currently saved.

Merges [25195] to the 3.6 branch.

props ocean90.
fixes #25064.