Dav: restored the request URI on a destination mapping error.

Dav: restored the request URI on a destination mapping error.

The COPY and MOVE handler temporarily replaces r->uri with the
destination to map it to a path, restoring it only on success.  When
ngx_http_map_uri_to_path() failed for the destination, the handler
returned without restoring r->uri, leaving it set to the client-supplied
destination.  Later processing that derives a target from the request
URI, such as an error_page with a variable, then resolved against the
destination instead of the request target.