Customizer logo function

gistfile1.php

<?php 
/**
 * Custom site logo function.
 *
 * Check if a site logo has been uploaded. If a site logo is present, query the database to retrieve
 * the medium sized version of the image. This function prevents excessively large logo images being
 * displayed on the frontend.
 */
function twentyfourteen_site_logo() {
	$site_logo = get_theme_mod( 'site_logo' );
        if ( $site_logo ) { 
		global $wpdb;
		$logo_query = "SELECT ID FROM {$wpdb->posts} WHERE guid='{$site_logo}'";
		$logo_id = $wpdb->get_var($logo_query);
		$logo = wp_get_attachment_image_src( $logo_id,'medium' );
		return $logo[0];
	}
}
?>