Call QEventDispatcher::startingUp() on thread start

...instead of during createEventDispatcher().

This way, startingUp() will be called [on the thread
being started] also for custom event dispatchers installed
with QThread::setEventDispatcher().

This prevents crashes when installing event dispatches which
expects that startingUp() will be called.

Crash reproducible with test case from QTBUG-51961.

Change-Id: I71bd696539689d928a61ff9b47d05297cf803b2f
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/thread/qthread.cpp

@@ -125,7 +125,6 @@ QAbstractEventDispatcher *QThreadData::createEventDispatcher()
 {
     QAbstractEventDispatcher *ed = QThreadPrivate::createEventDispatcher(this);
     eventDispatcher.storeRelease(ed);
-    ed->startingUp();
     return ed;
 }
 

src/corelib/thread/qthread_unix.cpp

@@ -308,6 +308,7 @@ void *QThreadPrivate::start(void *arg)
         }
 
         data->ensureEventDispatcher();
+        data->eventDispatcher.loadRelaxed()->startingUp();
 
 #if (defined(Q_OS_LINUX) || defined(Q_OS_MAC) || defined(Q_OS_QNX))
         {

src/corelib/thread/qthread_win.cpp

@@ -312,6 +312,7 @@ unsigned int __stdcall QT_ENSURE_STACK_ALIGNED_FOR_SSE QThreadPrivate::start(voi
     }
 
     data->ensureEventDispatcher();
+    data->eventDispatcher.loadRelaxed()->startingUp();
 
 #if !defined(QT_NO_DEBUG) && defined(Q_CC_MSVC)
     // sets the name of the current thread.