use apr_pstrndup() instead of apr_pstrdup() to avoid a strlen call in

ap_md5_binary, since we know the length of the string is always
MD5_DIGESTSIZE * 2
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git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@92212 13f79535-47bb-0310-9956-ffa450edef68