message: Make GetWitnessCommitmentIndex callable on blocks without a coinbase txn.
  This isn't actually needed anywhere, but it's less brittle.
  Github-Pull:
  Rebased-From:
diff: @@ -2922,11 +2922,13 @@ bool IsWitnessEnabled(const CBlockIndex* pindexPrev, const Consensus::Params& pa
  static int GetWitnessCommitmentIndex(const CBlock& block)
  {
  int commitpos = -1;
  + if (!block.vtx.empty()) {
  for (size_t o = 0; o < block.vtx[0]->vout.size(); o++) {
  if (block.vtx[0]->vout[o].scriptPubKey.size() >= 38 && block.vtx[0]->vout[o].scriptPubKey[0] == OP_RETURN && block.vtx[0]->vout[o].scriptPubKey[1] == 0x24 && block.vtx[0]->vout[o].scriptPubKey[2] == 0xaa && block.vtx[0]->vout[o].scriptPubKey[3] == 0x21 && block.vtx[0]->vout[o].scriptPubKey[4] == 0xa9 && block.vtx[0]->vout[o].scriptPubKey[5] == 0xed) {
  commitpos = o;
  }
  }
  + }
  return commitpos;
  }
  