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/ChildDef/Custom.php

@@ -45,7 +45,7 @@ class HTMLPurifier_ChildDef_Custom extends HTMLPurifier_ChildDef
     protected function _compileRegex()
     {
         $raw = str_replace(' ', '', $this->dtd_regex);
-        if ($raw{0} != '(') {
+        if ($raw[0] != '(') {
             $raw = "($raw)";
         }
         $el = '[#a-zA-Z0-9_.-]+';