Support single sign-on in wincred

Libcurl supports NTLM authentication using the Windows credentials of current user. To do that, username and password should be empty. Unfortunately, such credentials cause the wincred helper to crash. This patch corrects that and adds this scenario to credential helper tests.

I had previously [sent this patch to the mailing list](https://groups.google.com/d/topic/msysgit/SCVa92TjUP4/discussion), but the v2 version seems to have been lost in traffic, so I'm sending this as a pull request for better visibility.