http: Fix CSD https requests

If request_body is NULL, buf_error(request_body) will return -ENOMEM to
indicate an allocation failure.  This is a good thing if we just called
buf_alloc(), but it is a bad thing if we aren't actually passing in a
request_body pointer.

CSD GET requests were affected by this; fix it.

Signed-off-by: Kevin Cernekee <cernekee@gmail.com>