chain: Set all CBlockIndex members to null, remove SetNull helper
chain: Set all CBlockIndex members to null, remove SetNull helper

Former-commit-id: fd1ca4813f1e27da9fa1f1ce04b3d5ac91e76d24
syscoin/syscoin,syscoin/syscoin2,syscoin/syscoin,syscoin/syscoin2,syscoin/syscoin2,syscoin/syscoin2,syscoin/syscoin,syscoin/syscoin2,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin2