Prevent pegin double spends from entering the mempool

This bug got introduced with the 0.17 rebase. In 0.14.1 the double pegin
claim check was done in the beginning of the validation logic, where the
coins view was the actual UTXO set. In 0.17, Core moved several checks
because the intermediate method CheckInputs was removed and some logic
moved to tx_verify.cpp, where we also put the pegin check.  In the
mempool acceptance check, however (as opposed to the ConnectBlock
check), CheckTxInputs is called with only a partial UTXO view for
optimization reasons. This commit adds an extra pegin check in the
beginning of the mempool code where we have a full UTXO view.

src/validation.cpp

@@ -681,10 +681,27 @@ static bool AcceptToMemoryPoolWorker(const CChainParams& chainparams, CTxMemPool
         }
 
         // do all inputs exist?
-        for (const CTxIn& txin : tx.vin) {
+        for (unsigned int i = 0; i < tx.vin.size(); i++) {
+            const CTxIn& txin = tx.vin[i];
+
             // ELEMENTS:
-            // Don't look for coins that only exist in parent chain
+            // For pegin inputs check whether the pegins have already been claimed before.
+            // This only checks the UTXO set for already claimed pegins. For mempool conflicts,
+            // we rely on the GetConflictTx check done above.
             if (txin.m_is_pegin) {
+                // Quick sanity check on witness first.
+                if (tx.witness.vtxinwit.size() <= i ||
+                        tx.witness.vtxinwit[i].m_pegin_witness.stack.size() < 6 ||
+                        uint256(tx.witness.vtxinwit[i].m_pegin_witness.stack[2]).IsNull() ||
+                        tx.vin[i].prevout.hash.IsNull()) {
+                    return state.Invalid(false, REJECT_INVALID, "pegin-no-witness");
+                }
+
+                std::pair<uint256, COutPoint> pegin = std::make_pair(uint256(tx.witness.vtxinwit[i].m_pegin_witness.stack[2]), tx.vin[i].prevout);
+                // This assumes non-null prevout and genesis block hash
+                if (view.IsPeginSpent(pegin)) {
+                    return state.Invalid(false, REJECT_INVALID, "pegin-already-claimed");
+                }
                 continue;
             }