wp_kses_attr_check: add isset before accessing array key

[tristanleboss]

In file wp-includes/kses.php, the first line of the function wp_kses_attr_check tries to use a function argument directly as an array key without any check if it exists. This throws a notice in PHP if the key doesn't exists.

As this function is publicly available, I think it would be a cool idea to do some check before using the variable as an array key.

<?php
function wp_kses_attr_check( &$name, &$value, &$whole, $vless, $element, $allowed_html ) {
        $allowed_attr = $allowed_html[ strtolower( $element ) ];

[SergeyBiryukov]

In 46959:

Formatting: Return early from wp_kses_attr_check() if the element is not in the list of allowed elements and attributes.

Props krynes, tristanleboss.
Fixes #48549.