Fix error when signing with Swedish BankID card

Some Swedish BankID cards (at least card type 6006 and probably 6005) have key reference 0, but SC_SEC_ENV_KEY_REF_PRESENT should not be set since this causes an error when creating a signature (using fribid on Ubuntu x64). Also see http://en.it-usenet.org/thread/11873/12745/. This commit fixes #265