[mod_httpapi] Fix possible segfault when HEAD request fails

[udovichenkoAlexander]

When HEAD request fails with status code different from 404 NOT FOUND then NULL pointer passed to %s because save_path is NULL.

 "caching: url:%s to %s failed with HTTP response code %d\n", url, save_path, (int)code

I do some tests with my backend witch return 400 BAD REQUEST. Logs:

caching: url:... to (null) failed with HTTP response code 400

It's glibc behavior, but this is UB by standart and may lead to SegFault. StackOverflow Question