HTML API: Ensure that code points always encode to UTF-8

This was brought up during fuzz testing of the HTML API. After
polyfilling `mb_chr()` and relying on it in the HTML decoder, it became
possible that for sites with a non-UTF-8 charset selected, then the
creation of text from code points when decoding numeric character
references might produce corrupted text, or text which encodes to
non-UTF-8 bytes.

While for these sites, there are broader issues with non-UTF-8 support,
this change ensures that code point encoding remains deterministic.

Developed in: WordPress#12155
Discussed in: https://core.trac.wordpress.org/ticket/65372

Follow-up to [62424].

Props dmsnell, jonsurrell.
See #65372.


git-svn-id: https://develop.svn.wordpress.org/trunk@62487 602fd350-edb4-49c9-b593-d223f7449a82

Author: dmsnell

src/wp-includes/html-api/class-wp-html-decoder.php

@@ -424,7 +424,7 @@ public static function read_character_reference( $context, $text, $at = 0, &$mat
 	 * @return string Converted code point, or `�` if invalid.
 	 */
 	public static function code_point_to_utf8_bytes( $code_point ): string {
-		$string = mb_chr( $code_point );
+		$string = mb_chr( $code_point, 'UTF-8' );
 
 		return false !== $string ? $string : '�';
 	}