mod_rewrite: Handle per-location rules when r->filename is unset.

Previously this would segfault or simply not match as expected,
depending on the platform.
PR:
Obtained from:
Submitted by:	trawick
Reviewed by:	trawick, geoff, jerenkrantz


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/APACHE_2_0_BRANCH@105434 13f79535-47bb-0310-9956-ffa450edef68