Customizer: Prevent JS error during init when `nav_menus` panel is removed by plugin.

Cherry-picks [33753] onto 4.3 branch.
Fixes #33411 for 4.3.

Built from https://develop.svn.wordpress.org/branches/4.3@33943


git-svn-id: http://core.svn.wordpress.org/branches/4.3@33912 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-admin/js/customize-nav-menus.js

@@ -119,6 +119,10 @@
 		initialize: function() {
 			var self = this;
 
+			if ( ! api.panel.has( 'nav_menus' ) ) {
+				return;
+			}
+
 			this.$search = $( '#menu-items-search' );
 			this.sectionContent = this.$el.find( '.accordion-section-content' );