[Qt] Fix deprecated pixmap() return by pointer with Qt 5.15

Summary:
The `pixmap()` method returning a `QImage *` is deprecated with Qt 5.15.
It is replaced by a new `pixmap(Qt::ReturnByValue)` call which returns
by value as its name implies.

Test Plan:
  ninja all check

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D6573

src/qt/bitcoingui.cpp

@@ -1285,7 +1285,7 @@ void BitcoinGUI::updateProxyIcon() {
     bool proxy_enabled = clientModel->getProxyInfo(ip_port);
 
     if (proxy_enabled) {
-        if (labelProxyIcon->pixmap() == nullptr) {
+        if (!labelProxyIcon->hasPixmap()) {
             QString ip_port_q = QString::fromStdString(ip_port);
             labelProxyIcon->setPixmap(
                 platformStyle->SingleColorIcon(":/icons/proxy")

src/qt/guiutil.cpp

@@ -951,6 +951,14 @@ QString formatBytes(uint64_t bytes) {
     return QString(QObject::tr("%1 GB")).arg(bytes / 1024 / 1024 / 1024);
 }
 
+bool ClickableLabel::hasPixmap() const {
+#if (QT_VERSION >= QT_VERSION_CHECK(5, 15, 0))
+    return !pixmap(Qt::ReturnByValue).isNull();
+#else
+    return pixmap() != nullptr;
+#endif
+}
+
 void ClickableLabel::mouseReleaseEvent(QMouseEvent *event) {
     Q_EMIT clicked(event->pos());
 }

src/qt/guiutil.h

@@ -241,6 +241,9 @@ QString formatBytes(uint64_t bytes);
 class ClickableLabel : public QLabel {
     Q_OBJECT
 
+public:
+    bool hasPixmap() const;
+
 Q_SIGNALS:
     /** Emitted when the label is clicked. The relative mouse coordinates of the
      * click are passed to the signal.

src/qt/receiverequestdialog.cpp

@@ -40,15 +40,24 @@ QRImageWidget::QRImageWidget(QWidget *parent)
     contextMenu->addAction(copyImageAction);
 }
 
+bool QRImageWidget::hasPixmap() const {
+#if (QT_VERSION >= QT_VERSION_CHECK(5, 15, 0))
+    return !pixmap(Qt::ReturnByValue).isNull();
+#else
+    return pixmap() != nullptr;
+#endif
+}
+
 QImage QRImageWidget::exportImage() {
-    if (!pixmap()) {
-        return QImage();
-    }
-    return pixmap()->toImage();
+#if (QT_VERSION >= QT_VERSION_CHECK(5, 15, 0))
+    return pixmap(Qt::ReturnByValue).toImage();
+#else
+    return hasPixmap() ? pixmap()->toImage() : QImage();
+#endif
 }
 
 void QRImageWidget::mousePressEvent(QMouseEvent *event) {
-    if (event->button() == Qt::LeftButton && pixmap()) {
+    if (event->button() == Qt::LeftButton && hasPixmap()) {
         event->accept();
         QMimeData *mimeData = new QMimeData;
         mimeData->setImageData(exportImage());
@@ -62,7 +71,7 @@ void QRImageWidget::mousePressEvent(QMouseEvent *event) {
 }
 
 void QRImageWidget::saveImage() {
-    if (!pixmap()) {
+    if (!hasPixmap()) {
         return;
     }
     QString fn = GUIUtil::getSaveFileName(this, tr("Save QR Code"), QString(),
@@ -73,14 +82,14 @@ void QRImageWidget::saveImage() {
 }
 
 void QRImageWidget::copyImage() {
-    if (!pixmap()) {
+    if (!hasPixmap()) {
         return;
     }
     QApplication::clipboard()->setImage(exportImage());
 }
 
 void QRImageWidget::contextMenuEvent(QContextMenuEvent *event) {
-    if (!pixmap()) {
+    if (!hasPixmap()) {
         return;
     }
     contextMenu->exec(event->globalPos());

src/qt/receiverequestdialog.h

@@ -29,6 +29,7 @@ class QRImageWidget : public QLabel {
 
 public:
     explicit QRImageWidget(QWidget *parent = nullptr);
+    bool hasPixmap() const;
     QImage exportImage();
 
 public Q_SLOTS: