risc0-ethereum-contracts allows invalid commitment with digest value of zero to be accepted by Steel.validateCommitment

Summary

RISC Zero is a zero-knowledge verifiable general computing platform, with Ethereum integration. The risc0-ethereum repository contains Solidity verifier contracts, Steel EVM view call library, and supporting code. Prior to versions 2.1.1 and 2.2.0, the `Steel.validateCommitment` Solidity library function will return `true` for a crafted commitment with a digest value of zero. This violates the semantics of `validateCommitment`, as this does not commitment to a block that is in the current chain. Because the digest is zero, it does not correspond to any block and there exist no known openings. As a result, this commitment will never be produced by a correct zkVM guest using Steel and leveraging this bug to compromise the soundness of a program using Steel would require a separate bug or misuse of the Steel library, which is expected to be used to validate the root of state opening proofs. A fix has been released as part of `risc0-ethereum` 2.1.1 and 2.2.0. Users for the `Steel` Solidity library versions 2.1.0 or earlier should ensure they are using `Steel.validateCommitment` in tandem with zkVM proof verification of a Steel program, as shown in the ERC-20 counter example, and documentation. This is the correct usage of Steel, and users following this pattern are not at risk, and do not need to take action. Users not verifying a zkVM proof of a Steel program should update their application to do so, as this is incorrect usage of Steel.