Restore some assert semantics in sigop cost calculations

There are some similar asserts which are left removed in policy
and ATMP (policy code being broken isn't a huge deal, but if we
fail to verify some consensus rules, we should most definitely
crash).

src/consensus/tx_verify.cpp

@@ -126,7 +126,9 @@ unsigned int GetP2SHSigOpCount(const CTransaction& tx, const CCoinsViewCache& in
     unsigned int nSigOps = 0;
     for (unsigned int i = 0; i < tx.vin.size(); i++)
     {
-        const CTxOut &prevout = inputs.AccessCoin(tx.vin[i].prevout).out;
+        const Coin& coin = inputs.AccessCoin(tx.vin[i].prevout);
+        assert(!coin.IsSpent());
+        const CTxOut &prevout = coin.out;
         if (prevout.scriptPubKey.IsPayToScriptHash())
             nSigOps += prevout.scriptPubKey.GetSigOpCount(tx.vin[i].scriptSig);
     }
@@ -146,7 +148,9 @@ int64_t GetTransactionSigOpCost(const CTransaction& tx, const CCoinsViewCache& i
 
     for (unsigned int i = 0; i < tx.vin.size(); i++)
     {
-        const CTxOut &prevout = inputs.AccessCoin(tx.vin[i].prevout).out;
+        const Coin& coin = inputs.AccessCoin(tx.vin[i].prevout);
+        assert(!coin.IsSpent());
+        const CTxOut &prevout = coin.out;
         nSigOps += CountWitnessSigOps(tx.vin[i].scriptSig, prevout.scriptPubKey, &tx.vin[i].scriptWitness, flags);
     }
     return nSigOps;