Prevent errors occurring when there is no menu or link

[evokelektrique]

Today I tried this Walker and I didn't had any menus registered so I encountered an error in line 36 and 15, This pull request fixed those issues.

[mbrughi]

It is possible to avoid the error by checking that there is actually an assigned menu before trying to use the location:

		        if (has_nav_menu('primary')) {
			        wp_nav_menu( array(
			            'theme_location'    => 'primary',
			            'depth'             => 0,
			            'container'         => false,
			            //'items_wrap'     => 'div',
			            'menu_class'        => 'navbar-menu',
			            'menu_id'           => 'primary-menu',
			            'after'             => "</div>",
			            'walker'            => new Navwalker())
			        );
			    }