Drupal 6.32/7.30 affected by XML Quadratic Blowup

Wordpress prior to 3.9.2 is affected by the XML Quadratic Blowup vulnerability which is reflected in pyfiscan. However, Drupal core prior to version 6.33 and 7.31 is also affected by the same vulnerability. This pull request addresses that issue. Couldn't find a CVE reference for the vulnerability, hence that part is missing in the comments above the YAML definition.