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-admin/link-parse-opml.php

@@ -94,3 +94,4 @@ if ( ! xml_parse( $xml_parser, $opml, true ) ) {
 
 // Free up memory used by the XML parser.
 xml_parser_free( $xml_parser );
+unset( $xml_parser );