QReadWriteLock: fix another data race in stateForWaitCondition()

The writerCount member variable is only ever accessed under
Private::mutex protection, so we need to lock the mutex here, too.

Pick-to: 6.3 6.2 5.15
Change-Id: I1717e5282eed2ecc14ccdc5090b9fc41174cccc8
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/thread/qreadwritelock.cpp

@@ -422,6 +422,7 @@ QReadWriteLock::StateForWaitCondition QReadWriteLock::stateForWaitCondition() co
 
     if (!d)
         return Unlocked;
+    const auto lock = qt_scoped_lock(d->mutex);
     if (d->writerCount > 1)
         return RecursivelyLocked;
     else if (d->writerCount == 1)