Restore expected qApp->exit() behaviour when called from a non-GUI thread

src/kernel/qapplication.cpp

@@ -2931,7 +2931,14 @@ int QApplication::exec()
 */
 void QApplication::exit( int retcode )
 {
-    qApp->eventLoop()->exit( retcode );
+    QThread* thread = qApp->guiThread();
+    if (thread) {
+        if (thread->d) {
+            if (thread->d->eventLoop) {
+                thread->d->eventLoop->exit( retcode );
+            }
+        }
+    }
 }
 
 /*!