x509: fix return error code for failed decryption without key

x509: fix return error code for failed decryption without key

Decrypting an encrypted private key previously returned
GNUTLS_E_DECRYPTION_FAILED when no password was supplied.  This changed when
decryption via pin callbacks was added in d31b89de.

That change should have included a check for callback existence in order to
preserve the error path of the no-password case.

This adds the check and a test for the previous behavior.
Resolves bug #1321

Signed-off-by: Craig Gallek <cgallek@gmail.com>