Work-around crash in QThreadPool QThread usage

This works around mismatch in threads starting and restarting QThreads,
and is safe since we don't need to establish a binding, and objectName
access in QThreadPool is locked behind a mutex.

Pick-to: 6.3 6.2
Fixes: QTBUG-96718
Change-Id: Id3f75e4f8344796ca658899645219fe3373ddd6d
Reviewed-by: Marc Mutz <marc.mutz@qt.io>

src/corelib/thread/qthread_unix.cpp

@@ -735,7 +735,9 @@ void QThread::start(Priority priority)
     else
         pthread_attr_setthreadname(&attr, objectName().toLocal8Bit());
 #else
-    d->objectName = objectName();
+    // avoid interacting with the binding system
+    d->objectName = d->extraData ? d->extraData->objectName.valueBypassingBindings()
+                                 : QString();
 #endif
 
     pthread_t threadId;

src/corelib/thread/qthread_win.cpp

@@ -420,7 +420,9 @@ void QThread::start(Priority priority)
     if (d->running)
         return;
 
-    d->objectName = objectName();
+    // avoid interacting with the binding system
+    d->objectName = d->extraData ? d->extraData->objectName.valueBypassingBindings()
+                                 : QString();
     d->running = true;
     d->finished = false;
     d->exited = false;