mod_proxy: Don't mangle r->filename when ap_proxy_canon_netloc() fails.

mod_proxy: Don't mangle r->filename when ap_proxy_canon_netloc() fails.

ap_proxy_canon_netloc() called from canon_handler hooks modifies its given
url in pace, hence &r->filename[6] passed from ap_proxy_canon_url().

This is not an issue if every canon_handler hook succeeds (or declines)
since r->filename is usually completely rewritten finally, but on failure
it gets truncated.

Avoid this by passing a copy of r->filename from the start, the proxy *url
and r->filename don't need to point to the same data.

* proxy/proxy_util.c(ap_proxy_canon_url):
  Pass a copy of r->filename to the canon_handler hooks.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1919023 13f79535-47bb-0310-9956-ffa450edef68