Fix error Gravatar alt

You don’t check anywhere that you’re viewing a comment when you use get_comment_author();. The get_avatar() function is used in a lot of places in WordPress; your code seems to assume it’s only in use on comments.

Try this (the code is untested, but should work, I think):

add_filter( 'get_avatar' , 'alt_name_avatar');
function alt_name_avatar( $avatar ) {
    if ( null === get_comment() ) {
        // This isn't a comment.
        return $avatar;
    }
    $alt = get_comment_author();
    $avatar = str_replace('alt=\'\'','alt=\'Avatar for '.$alt.'\' title=\'Avatar for '.$alt.'\'',$avatar);
    return $avatar;
}

There doesn’t seem to be a simple is_comment() check to see if we’re viewing a comment, so I’ve chosen to test get_comment(), which will return null if we’re not in a comment.