Make GetWitnessCommitmentIndex callable on blocks without a coinbase txn.

This isn't actually needed anywhere, but it's less brittle.

Github-Pull: #10146
Rebased-From: ada0caa165

src/validation.cpp

@@ -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;
 }