Provide a better return value from perl_parse_require_line() in the case where interp is NULL and add a comment to explain why we currently don't expect this to happen.

Thanks again to Jeff Trawick for this.

git-svn-id: https://svn.apache.org/repos/asf/perl/modperl/branches/httpd24threading@1539487 13f79535-47bb-0310-9956-ffa450edef68