GHSA-X39J-H85H-3F46 go-merkledag's ProtoNode may be modified such that common method calls may panic

Impact A ProtoNode may be modified in such a way as to cause various encode errors which will trigger a panic on common method calls that don't allow for error returns. A ProtoNode should only be able to encode to valid DAG-PB, attempting to encode invalid DAG-PB forms will result in an error fro...