Avoid memory leak by registering shutdown function exactly 1 time

Fixes #2092

src/PhpSpreadsheet/Reader/Security/XmlScanner.php

@@ -18,6 +18,11 @@ class XmlScanner
 
     private static $libxmlDisableEntityLoaderValue;
 
+    /**
+     * @var bool
+     */
+    private static $shutdownRegistered = false;
+
     public function __construct($pattern = '<!DOCTYPE')
     {
         $this->pattern = $pattern;
@@ -25,7 +30,10 @@ class XmlScanner
         $this->disableEntityLoaderCheck();
 
         // A fatal error will bypass the destructor, so we register a shutdown here
-        register_shutdown_function([__CLASS__, 'shutdown']);
+        if (!self::$shutdownRegistered) {
+            self::$shutdownRegistered = true;
+            register_shutdown_function([__CLASS__, 'shutdown']);
+        }
     }
 
     public static function getInstance(Reader\IReader $reader)