ui: Check for !pixmap() before trying to export QR code

Adds null pointer checks as well as prevents the Save/Copy context
menu from appearing at all if no image is shown.

Fixes issue #4140

src/qt/receiverequestdialog.h

@@ -15,6 +15,9 @@ namespace Ui {
     class ReceiveRequestDialog;
 }
 class OptionsModel;
+QT_BEGIN_NAMESPACE
+class QMenu;
+QT_END_NAMESPACE
 
 /* Label widget for QR code. This image can be dragged, dropped, copied and saved
  * to disk.
@@ -33,6 +36,10 @@ public slots:
 
 protected:
     virtual void mousePressEvent(QMouseEvent *event);
+    virtual void contextMenuEvent(QContextMenuEvent *event);
+
+private:
+    QMenu *contextMenu;
 };
 
 class ReceiveRequestDialog : public QDialog