Fix null pointer dereference in NTLM authentication

Fix null pointer dereference in NTLM authentication

If NTLM authentication is required for the URL with an empty path,
then QNetworkAuthenticationCache::findClosestMatch(url.path()) returns 0.
e.g. "http://10.1.2.3".
Return a default constructed credential in this case.

Change-Id: I84ad3b308ee3f74fbbac9ad0f11dbdc66047b50b
Reviewed-by: Robin Burchell <robin+qt@viroteck.net>
Reviewed-by: Martin Petersson <Martin.Petersson@nokia.com>