prevent near infinite subrequest recursion with mod_negotiation enabled.

This can happen if there is a partial match between a bad URI and a
file with a variant extention.

ap_sub_req_lookup_dirent has apparently been generating bogus subrequest
URIs for ages, but they used to be ignored.  Once we started calling
ap_process_request_internal for all subrequests, they started causing
problems.  Make it explicit that rnew->uri is to be ignored for this type
of subrequest.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@91375 13f79535-47bb-0310-9956-ffa450edef68