uhttpd/file: fix string out of buffer range on uh_defer_script

uhttpd/file: fix string out of buffer range on uh_defer_script

if a url path length is multiple of 8, tailing zero will be trimed out on uh_defer_script, cause a strangle error.
it's simple to reproduce.

1. create a luci controller, register a entry with path length multiple of 8 (including '/cgi-bin/'), for example, '/cgi-bin/luci/admin/system/admin'.
2. set uhttpd max_requests to 1, and restart uhttpd
3. request '/cgi-bin/luci/admin/system/admin' with at least 2 process
4. some responses will produce a error:
```
Unable to launch the requested CGI program:
  /www/cgi-bin/luci: No such file or directory
```

Signed-off-by: Liangbin Lian <jjm2473@gmail.com>