fix: [stix2 export] Avoiding the "end" function to return a notice

- It looks like depending on the Php version, the
  end function does not like to have the reference
  of an array. By delaying its call, we pass the
  actual array and the notice no longer appears

app/Lib/Export/Stix2Export.php

@@ -20,7 +20,7 @@ class Stix2Export extends StixExport
         $filename = $this->__scripts_dir . 'tmp/' . $filename;
         $my_server = ClassRegistry::init('Server');
         $result = shell_exec($my_server->getPythonVersion() . ' ' . $scriptFile . ' ' . $filename . $this->__end_of_cmd);
-        $result = end(preg_split("/\r\n|\n|\r/", trim($result)));
-        return $result;
+        $result = preg_split("/\r\n|\n|\r/", trim($result));
+        return end($result);
     }
 }