MDL-67030 htmlpurifier: php74 fixup for htmlpurifier v4.10.0

This simply applies this pull request:

https://github.com/ezyang/htmlpurifier/pull/224

straight to our library. Once there is some new htmlpurifier
release available (>v4.11.0) we'll be back upstream because
the fix will be included.

lib/htmlpurifier/HTMLPurifier/Encoder.php

@@ -159,7 +159,7 @@ class HTMLPurifier_Encoder
 
         $len = strlen($str);
         for ($i = 0; $i < $len; $i++) {
-            $in = ord($str{$i});
+            $in = ord($str[$i]);
             $char .= $str[$i]; // append byte to char
             if (0 == $mState) {
                 // When mState is zero we expect either a US-ASCII character