Proxy: fixed segfault in URI change.

Proxy: fixed segfault in URI change.

If request URI was shorter than location prefix, as after replacement
with try_files, location length was used to copy the remaining URI part
leading to buffer overread.

The fix is to replace full request URI in this case.  In the following
configuration, request "/123" is changed to "/" when sent to backend.

    location /1234 {
        try_files /123 =404;
        proxy_pass http://127.0.0.1:8080/;
    }

Closes #983 on GitHub.