try this
<?php
$logo_id = get_theme_mod( 'custom_logo' );
$logo = wp_get_attachment_image_src( $logo_id , 'full' );
$logo_path = get_template_directory_uri() . "/img/logo.png";
if( has_custom_logo() ) :
echo "<a href='". esc_url( home_url( '/' ) ) ."'>";
echo "<img src='". esc_url( $logo[0] ) ."' /></a>";
else :
echo "<a href='". esc_url( home_url( '/' ) ) ."'>";
echo "<img src='". esc_url( $logo_path ) ."' /></a>";
endif;
?>