Fortify falsely complained that the sprintf() result was unbounded.

We may as well use apr_snprintf() though, as well as comment on the
available space for "::\0" (for people like me).


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@987498 13f79535-47bb-0310-9956-ffa450edef68