fix sys burn to asset, 0 val logic
fix sys burn to asset, 0 val logic

should match checksyscoinmint logic


Former-commit-id: 91eb75b8f3f6ae448695cbb0d2505dd23c4fb7bb
syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin