One more fix for RewriteMap programs. 0 when treated as NULL is ok as an error

condition for filehandles (FILE *) but not for filenumbers. Here we have to
use -1 as the error indicator like open() does.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/1.3.x@80325 13f79535-47bb-0310-9956-ffa450edef68