Avoid zero-as-nullpointer warnings in QThread

Change-Id: I3fd557a54d63c2dcabe58fab65326538896d02a2
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Marc Mutz <marc.mutz@kdab.com>

src/corelib/thread/qthread.h

@@ -140,10 +140,10 @@ public:
     static QThread* currentThread();
 
 protected:
-    QThread(QThreadPrivate &dd, QObject *parent = 0);
+    QThread(QThreadPrivate &dd, QObject *parent = nullptr);
 
 private:
-    explicit QThread(QObject *parent = 0);
+    explicit QThread(QObject *parent = nullptr);
     static QThread *instance;
 
     friend class QCoreApplication;