Do not access internal allThreads data unlocked

Pick-to: 6.1 6.0 5.15
Change-Id: I54eb67571fff07ffdbf9d2b77c96bb85e3fae5e0
Reviewed-by: Sona Kurazyan <sona.kurazyan@qt.io>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/thread/qthreadpool.cpp

@@ -788,6 +788,7 @@ bool QThreadPool::contains(const QThread *thread) const
     const QThreadPoolThread *poolThread = qobject_cast<const QThreadPoolThread *>(thread);
     if (!poolThread)
         return false;
+    QMutexLocker locker(&d->mutex);
     return d->allThreads.contains(const_cast<QThreadPoolThread *>(poolThread));
 }