wallet, refactor: Immutable CWalletTx::pwallet

src/wallet/wallet.h

@@ -275,7 +275,7 @@ int CalculateMaximumSignedInputSize(const CTxOut& txout, const CWallet* pwallet,
 class CWalletTx
 {
 private:
-    const CWallet* pwallet;
+    const CWallet* const pwallet;
 
     /** Constant used in hashBlock to indicate tx has been abandoned, only used at
      * serialization/deserialization to avoid ambiguity with conflicted.