Get URL of the custom logo in WordPress.

get-logo.php

<?php
/**
 * Get theme logo URL
 */
function xx_get_logo() {
	$custom_logo_id = get_theme_mod( 'custom_logo' );
	if ( ! $custom_logo_id )
		return;

	$image = wp_get_attachment_image_src( $custom_logo_id , 'full' );
	$url = $image[0];

	return $url;
}