Unchecked input in HTMLPurifier_UnitConverter round() function

Hello, I found an issue in the variable checking in round() function of the HTMLPurifier_UnitConverter.

If, for some reason, round() is called with an invalid value, it crashes.
There is no check that the passed value $n is really a number, before trying to do abs($n).

    private function round($n, $sigfigs)
    {
               $new_log = (int)floor(log(abs($n), 10)); // Number of digits left of decimal - 1
    ...

At the moment, I temporary fixed my issue by patching the function with a check:

    private function round($n, $sigfigs)
    {
        if (!is_numeric($n)) { return $n; }
        $new_log = (int)floor(log(abs($n), 10)); // Number of digits left of decimal - 1
    ...

This is the stack trace of the call that generated the issue:

PHP Fatal error:  Uncaught TypeError: abs(): Argument #1 ($num) must be of type int|float, string given in /web/admin/vendor/ezyang/htmlpurifier/library/HTMLPurifier/UnitConverter.php:264
Stack trace:
#0 /web/admin/vendor/ezyang/htmlpurifier/library/HTMLPurifier/UnitConverter.php(264): abs()
#1 /web/admin/vendor/ezyang/htmlpurifier/library/HTMLPurifier/UnitConverter.php(178): HTMLPurifier_UnitConverter->round()
#2 /web/admin/vendor/ezyang/htmlpurifier/library/HTMLPurifier/Length.php(153): HTMLPurifier_UnitConverter->convert()
#3 /web/admin/vendor/ezyang/htmlpurifier/library/HTMLPurifier/AttrDef/CSS/Length.php(65): HTMLPurifier_Length->compareTo()
#4 /web/admin/vendor/ezyang/htmlpurifier/library/HTMLPurifier/AttrDef/CSS/Composite.php(39): HTMLPurifier_AttrDef_CSS_Length->validate()

The affected code is library/HTMLPurifier/UnitConverter.php, around line 264, in the private round() function. Read the call path shown through convert() and compareTo(), then verify that invalid input no longer reaches abs() and that normal conversion behavior remains intact.