qt: use CHashWriter also in SignVerifyMessageDialog

This eliminates unnecessary copying of data being hashed.

src/qt/signverifymessagedialog.cpp

@@ -148,12 +148,12 @@ void SignVerifyMessageDialog::on_signMessageButton_SM_clicked()
         return;
     }
 
-    CDataStream ss(SER_GETHASH, 0);
+    CHashWriter ss(SER_GETHASH, 0);
     ss << strMessageMagic;
     ss << ui->messageIn_SM->document()->toPlainText().toStdString();
 
     std::vector<unsigned char> vchSig;
-    if (!key.SignCompact(Hash(ss.begin(), ss.end()), vchSig))
+    if (!key.SignCompact(ss.GetHash(), vchSig))
     {
         ui->statusLabel_SM->setStyleSheet("QLabel { color: red; }");
         ui->statusLabel_SM->setText(QString("<nobr>") + tr("Message signing failed.") + QString("</nobr>"));
@@ -223,12 +223,12 @@ void SignVerifyMessageDialog::on_verifyMessageButton_VM_clicked()
         return;
     }
 
-    CDataStream ss(SER_GETHASH, 0);
+    CHashWriter ss(SER_GETHASH, 0);
     ss << strMessageMagic;
     ss << ui->messageIn_VM->document()->toPlainText().toStdString();
 
     CPubKey pubkey;
-    if (!pubkey.RecoverCompact(Hash(ss.begin(), ss.end()), vchSig))
+    if (!pubkey.RecoverCompact(ss.GetHash(), vchSig))
     {
         ui->signatureIn_VM->setValid(false);
         ui->statusLabel_VM->setStyleSheet("QLabel { color: red; }");