Conditional/Check: IF comment author is the post author

<?php if ( $post = get_post($post_id) ) : ?>
<?php if ( $comment->user_id === $post->post_author ) :?>
//do something
<?php endif;?>
<?php endif;?>

Also.

<?php 
global $post;
if( $comment->user_id === $post->post_author ) {
// do something
}
?>