In kerberos_sspi.cc you check for user to be null .... but it never is because it is the results of a std::string c_str() call. So, instead of saying:

if (user) {

you need to say

if (ulen > 0){

The following 2 lines need modifying:
  - https://github.com/mongodb-js/kerberos/blob/master/src/win32/kerberos_sspi.cc#L71
  - https://github.com/mongodb-js/kerberos/blob/master/src/win32/kerberos_sspi.cc#L95