Optimize GetWitnessHash() for non-segwit transactions

Github-Pull: #9912
Rebased-From: 02c57b521a

src/primitives/transaction.cpp

@@ -69,6 +69,9 @@ uint256 CTransaction::ComputeHash() const
 
 uint256 CTransaction::GetWitnessHash() const
 {
+    if (!HasWitness()) {
+        return GetHash();
+    }
     return SerializeHash(*this, SER_GETHASH, 0);
 }