nothread: do not delete reader thread object before it gets used

exec is an empty function on nothread and returns right away, so the
pixmap reader thread object gets deleted before it can download the url

Task-number: QTBUG-72105
Change-Id: If205dbe12d0b5299b43a68e7d0d955a2b89b4af1
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>
Reviewed-by: Morten Johan Sørvig <morten.sorvig@qt.io>

src/quick/util/qquickpixmapcache.cpp

@@ -471,6 +471,10 @@ QQuickPixmapReader::QQuickPixmapReader(QQmlEngine *eng)
     eventLoopQuitHack->moveToThread(this);
     connect(eventLoopQuitHack, SIGNAL(destroyed(QObject*)), SLOT(quit()), Qt::DirectConnection);
     start(QThread::LowestPriority);
+#if !QT_CONFIG(thread)
+    // call nonblocking run ourself, as nothread qthread does not
+    run();
+#endif
 }
 
 QQuickPixmapReader::~QQuickPixmapReader()
@@ -948,8 +952,11 @@ void QQuickPixmapReader::run()
     processJobs();
     exec();
 
+#if QT_CONFIG(thread)
+    // nothread exec is empty and returns
     delete threadObject;
     threadObject = nullptr;
+#endif
 }
 
 class QQuickPixmapKey