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/rss.php

@@ -94,6 +94,7 @@ class MagpieRSS {
 		}
 
 		xml_parser_free( $this->parser );
+		unset( $this->parser );
 
 		$this->normalize();
 	}