message: don't generate coinbase commitment for pos blocks
diff: @@ -2977,7 +2977,7 @@ std::vector<unsigned char> GenerateCoinbaseCommitment(CBlock& block, const CBloc
  std::vector<unsigned char> commitment;
  int commitpos = GetWitnessCommitmentIndex(block);
  std::vector<unsigned char> ret(32, 0x00);
  - if (commitpos == -1) {
  + if (block.IsProofOfWork() && commitpos == -1) {
  uint256 witnessroot = BlockWitnessMerkleRoot(block, nullptr);
  CHash256().Write(witnessroot.begin(), 32).Write(ret.data(), 32).Finalize(witnessroot.begin());
  CTxOut out;
  