crypt32: CryptAcquireCertificatePrivateKey should look up certificate in the user's store if not linked to a provider.

Under Windows passing to CryptAcquireCertificatePrivateKey() a certificate
retrieved from the key container using CryptGetKeyParam(KP_CERTIFICATE) +
CertCreateCertificateContext() works although such a certificate doesn't
have assigned properties at all. In order for this to work Windows probably
looks up an original certificate in the user's private store.
Signed-off-by: Dmitry Timoshkov <dmitry@baikal.ru>
Signed-off-by: Alexandre Julliard <julliard@winehq.org>