Fix race condition on qt_create_tls() on Windows

If this function is called by multiple threads, more than one could
reach the mutex locking and call TlsAlloc(), but only the last one would
save the data. The others would be leaked and, worse, be used by those
other threads.

[ChangeLog][QtCore][QObject] Fixed a resource leak caused by a race
condition if multiple QObjects were created at the same time, for the
first time in an application, from multiple threads (implies threads not
started with QThread).

Fixes: QTBUG-77238
Change-Id: Ife213d861bb14c1787e1fffd15b63a5818bcc807
Reviewed-by: Marc Mutz <marc.mutz@kdab.com>
Reviewed-by: Olivier Goffart (Woboq GmbH) <ogoffart@woboq.com>

src/corelib/thread/qthread_win.cpp

@@ -99,6 +99,8 @@ void qt_create_tls()
         return;
     static QBasicMutex mutex;
     QMutexLocker locker(&mutex);
+    if (qt_current_thread_data_tls_index != TLS_OUT_OF_INDEXES)
+        return;
     qt_current_thread_data_tls_index = TlsAlloc();
 }