Silence uninited variable compiler warnings

The code is correct but some compilers may still warn about dragData not
getting initialized. Silence them.

Change-Id: Ib52321667fc5094e22ebbef538b72b5477e6f10b
Reviewed-by: Giulio Camuffo <giulio.camuffo@jollamobile.com>

src/plugins/platforms/wayland/qwaylanddatadevice.cpp

@@ -126,7 +126,7 @@ void QWaylandDataDevice::data_device_drop()
 
     qDebug() << Q_FUNC_INFO << drag << m_dragOffer.data();
 
-    QMimeData *dragData;
+    QMimeData *dragData = 0;
     Qt::DropActions supportedActions;
     if (drag) {
         dragData = drag->mimeData();