crypto.c: check that the password is correct for login

With PKCS11 tokens, if C_Login returns OK that confirms the password
is correct.  However, with engine keys, the password may or may not be
checked in ENGINE_load_private_key() so try to perform a signing
operation if that function returns a key.

Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>