Return a bool in SpendCoin to restore pre-per-utxo assert semantics

Since its free to do so, assert that Spends succeeded when we expect
them to.

src/coins.cpp

@@ -91,9 +91,9 @@ void AddCoins(CCoinsViewCache& cache, const CTransaction &tx, int nHeight) {
     }
 }
 
-void CCoinsViewCache::SpendCoin(const COutPoint &outpoint, Coin* moveout) {
+bool CCoinsViewCache::SpendCoin(const COutPoint &outpoint, Coin* moveout) {
     CCoinsMap::iterator it = FetchCoin(outpoint);
-    if (it == cacheCoins.end()) return;
+    if (it == cacheCoins.end()) return false;
     cachedCoinsUsage -= it->second.coin.DynamicMemoryUsage();
     if (moveout) {
         *moveout = std::move(it->second.coin);
@@ -104,6 +104,7 @@ void CCoinsViewCache::SpendCoin(const COutPoint &outpoint, Coin* moveout) {
         it->second.flags |= CCoinsCacheEntry::DIRTY;
         it->second.coin.Clear();
     }
+    return true;
 }
 
 static const Coin coinEmpty;

src/coins.h

@@ -238,7 +238,7 @@ public:
      * If no unspent output exists for the passed outpoint, this call
      * has no effect.
      */
-    void SpendCoin(const COutPoint &outpoint, Coin* moveto = nullptr);
+    bool SpendCoin(const COutPoint &outpoint, Coin* moveto = nullptr);
 
     /**
      * Push the modifications applied to this cache to its base.

src/validation.cpp

@@ -1118,7 +1118,8 @@ void UpdateCoins(const CTransaction& tx, CCoinsViewCache& inputs, CTxUndo &txund
         txundo.vprevout.reserve(tx.vin.size());
         BOOST_FOREACH(const CTxIn &txin, tx.vin) {
             txundo.vprevout.emplace_back();
-            inputs.SpendCoin(txin.prevout, &txundo.vprevout.back());
+            bool is_spent = inputs.SpendCoin(txin.prevout, &txundo.vprevout.back());
+            assert(is_spent);
         }
     }
     // add outputs
@@ -1358,8 +1359,8 @@ static DisconnectResult DisconnectBlock(const CBlock& block, const CBlockIndex*
             if (!tx.vout[o].scriptPubKey.IsUnspendable()) {
                 COutPoint out(hash, o);
                 Coin coin;
-                view.SpendCoin(out, &coin);
-                if (tx.vout[o] != coin.out) {
+                bool is_spent = view.SpendCoin(out, &coin);
+                if (!is_spent || tx.vout[o] != coin.out) {
                     fClean = false; // transaction output mismatch
                 }
             }