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.

git-svn-id: https://develop.svn.wordpress.org/trunk@48322 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/atomlib.php

@@ -182,6 +182,7 @@ class AtomParser {
         fclose($fp);
 
         xml_parser_free($parser);
+        unset($parser);
 
         restore_error_handler();