qt: askpassphrasedialog: Clear pass fields on accept

This is usability improvement in a case if user gets re-asked
passphrase. (e.g. made a typo)

src/qt/askpassphrasedialog.cpp

@@ -77,10 +77,7 @@ AskPassphraseDialog::AskPassphraseDialog(Mode mode, QWidget *parent) :
 
 AskPassphraseDialog::~AskPassphraseDialog()
 {
-    // Attempt to overwrite text so that they do not linger around in memory
+    secureClearPassFields();
-    ui->passEdit1->setText(QString(" ").repeated(ui->passEdit1->text().size()));
-    ui->passEdit2->setText(QString(" ").repeated(ui->passEdit2->text().size()));
-    ui->passEdit3->setText(QString(" ").repeated(ui->passEdit3->text().size()));
     delete ui;
 }
 
@@ -103,6 +100,8 @@ void AskPassphraseDialog::accept()
     newpass1.assign(ui->passEdit2->text().toStdString().c_str());
     newpass2.assign(ui->passEdit3->text().toStdString().c_str());
 
+    secureClearPassFields();
+
     switch(mode)
     {
     case Encrypt: {
@@ -260,3 +259,17 @@ bool AskPassphraseDialog::eventFilter(QObject *object, QEvent *event)
     }
     return QDialog::eventFilter(object, event);
 }
+
+static void SecureClearQLineEdit(QLineEdit* edit)
+{
+    // Attempt to overwrite text so that they do not linger around in memory
+    edit->setText(QString(" ").repeated(edit->text().size()));
+    edit->clear();
+}
+
+void AskPassphraseDialog::secureClearPassFields()
+{
+    SecureClearQLineEdit(ui->passEdit1);
+    SecureClearQLineEdit(ui->passEdit2);
+    SecureClearQLineEdit(ui->passEdit3);
+}

src/qt/askpassphrasedialog.h

@@ -42,6 +42,7 @@ private:
 
 private Q_SLOTS:
     void textChanged();
+    void secureClearPassFields();
 
 protected:
     bool event(QEvent *event);