Disallow extended encoding for non-witness transactions

src/primitives/transaction.h

@@ -220,6 +220,10 @@ inline void UnserializeTransaction(TxType& tx, Stream& s) {
         for (size_t i = 0; i < tx.vin.size(); i++) {
             s >> tx.vin[i].scriptWitness.stack;
         }
+        if (!tx.HasWitness()) {
+            /* It's illegal to encode witnesses when all witness stacks are empty. */
+            throw std::ios_base::failure("Superfluous witness record");
+        }
     }
     if (flags) {
         /* Unknown flag in the serialization */