Panic in github.com/ipfs/go-merkledag

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.

Additionally, use of the ProtoNode.SetCidBuilder() method to set non-functioning CidBuilder (such as one that refers to a multihash where an implementation of that hash function is not available) may cause the same methods to panic as a new CID is required but cannot be created.