txdb: assert CCoinsViewDB::GetCoin only returns unspent coins

The chainstate UTXO database only stores unspent outputs; spent entries are removed.

Assert after reading a `Coin` so corruption or misuse cannot propagate a spent coin through the `GetCoin()` interface.

src/txdb.cpp

@@ -67,7 +67,10 @@ void CCoinsViewDB::ResizeCache(size_t new_cache_size)
 
 std::optional<Coin> CCoinsViewDB::GetCoin(const COutPoint& outpoint) const
 {
-    if (Coin coin; m_db->Read(CoinEntry(&outpoint), coin)) return coin;
+    if (Coin coin; m_db->Read(CoinEntry(&outpoint), coin)) {
+        Assert(!coin.IsSpent()); // The UTXO database should never contain spent coins
+        return coin;
+    }
     return std::nullopt;
 }