Fix deletion order in QImageReader/Writer destructors

The device would be deleted before the image format handler, and hence
be a dangling pointer that could easily cause a crash if the handler
or codec would access it on destruction, e.g. for cleanup.

Pick-to: 6.5 6.4 6.2 5.15
Change-Id: I51d16b1feddc5945955ac75a2e8701233dba7b82
Reviewed-by: Paul Olav Tvete <paul.tvete@qt.io>

src/gui/image/qimagereader.cpp

@@ -484,9 +484,9 @@ QImageReaderPrivate::QImageReaderPrivate(QImageReader *qq)
 */
 QImageReaderPrivate::~QImageReaderPrivate()
 {
+    delete handler;
     if (deleteDevice)
         delete device;
-    delete handler;
 }
 
 /*!

src/gui/image/qimagewriter.cpp

@@ -315,9 +315,9 @@ QImageWriter::QImageWriter(const QString &fileName, const QByteArray &format)
 */
 QImageWriter::~QImageWriter()
 {
+    delete d->handler;
     if (d->deleteDevice)
         delete d->device;
-    delete d->handler;
     delete d;
 }