XML-RPC: Explicitly unset the reference to the parser resource after calling xml_parser_free() to avoid memory leaks in PHP 7.0.0 or higher.

Props procifer, ayeshrajans.
Fixes #49700.
Built from https://develop.svn.wordpress.org/trunk@48322


git-svn-id: http://core.svn.wordpress.org/trunk@48091 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/IXR/class-IXR-message.php

@@ -110,20 +110,28 @@ class IXR_Message
         $chunk_size = apply_filters( 'xmlrpc_chunk_parsing_size', $chunk_size );
 
         $final = false;
+
         do {
             if (strlen($this->message) <= $chunk_size) {
                 $final = true;
             }
+
             $part = substr($this->message, 0, $chunk_size);
             $this->message = substr($this->message, $chunk_size);
+
             if (!xml_parse($this->_parser, $part, $final)) {
+                xml_parser_free($this->_parser);
+                unset($this->_parser);
                 return false;
             }
+
             if ($final) {
                 break;
             }
         } while (true);
+
         xml_parser_free($this->_parser);
+        unset($this->_parser);
 
         // Grab the error messages, if any
         if ($this->messageType == 'fault') {