Customize: Ensure that `WP_Customize_Nav_Menu_Section` is able to represent a placeholder nav menu.

Placeholder nav menus (ones that have yet to be saved into the DB) have negative integer IDs.

Fixes #37293.
Props valendesigns, stubgo, westonruter.

Built from https://develop.svn.wordpress.org/trunk@37981


git-svn-id: http://core.svn.wordpress.org/trunk@37922 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/customize/class-wp-customize-nav-menu-section.php

@@ -36,7 +36,7 @@ class WP_Customize_Nav_Menu_Section extends WP_Customize_Section {
 	 */
 	public function json() {
 		$exported = parent::json();
-		$exported['menu_id'] = intval( preg_replace( '/^nav_menu\[(\d+)\]/', '$1', $this->id ) );
+		$exported['menu_id'] = intval( preg_replace( '/^nav_menu\[(-?\d+)\]/', '$1', $this->id ) );
 
 		return $exported;
 	}

wp-includes/version.php

@@ -4,7 +4,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '4.6-beta1-37980';
+$wp_version = '4.6-beta1-37981';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.