MDL-76838 backup: remove invalid utf8 code sequences writing XML.

The surrogate blocks FFFE and FFFF are non-characters, specifically
disallowed in the character range for XML documents.

backup/util/xml/xml_writer.class.php

@@ -262,7 +262,7 @@ class xml_writer {
     protected function xml_safe_utf8($content) {
         $content = preg_replace('/[\x-\x8\xb-\xc\xe-\x1f\x7f]/is', '', $content ?? ''); // clean CTRL chars.
         $content = preg_replace("/\r\n|\r/", "\n", $content); // Normalize line&return=>line
-        return $content;
+        return fix_utf8($content);
     }
 
     /**

lib/moodlelib.php

@@ -1318,8 +1318,9 @@ function fix_utf8($value) {
             // Shortcut.
             return $value;
         }
-        // No null bytes expected in our data, so let's remove it.
-        $value = str_replace("\0", '', $value);
+
+        // Remove null bytes or invalid Unicode sequences from value.
+        $value = str_replace(["\0", "\xef\xbf\xbe", "\xef\xbf\xbf"], '', $value);
 
         // Note: this duplicates min_fix_utf8() intentionally.
         static $buggyiconv = null;