Don't expect QCoreApplictionPrivate::eventDispatcher to be set

QCoreApplication::setEventDispatcher sets the dispatcher on the current
thread, not on QCoreApplictionPrivate, so when running init() we don't
have an event dispatcher set.

Change-Id: Ia008e68b70777779ab14f1f7b9eeadac9adbcf7b
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>

src/corelib/kernel/qcoreapplication.cpp

@@ -848,8 +848,9 @@ void QCoreApplicationPrivate::init()
 
 #ifndef QT_NO_QOBJECT
     // use the event dispatcher created by the app programmer (if any)
-    if (!eventDispatcher)
-        eventDispatcher = threadData->eventDispatcher.load();
+    Q_ASSERT(!eventDispatcher);
+    eventDispatcher = threadData->eventDispatcher.load();
+
     // otherwise we create one
     if (!eventDispatcher)
         createEventDispatcher();