Add reference to CVE-2012-0021, quoting the 2.3.x version in which

the problem was introduced.

(following r1235875 which DidTRT for 2.2.x/CHANGES)


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1235894 13f79535-47bb-0310-9956-ffa450edef68