init mTouchDevice to null

Since QWaylandInputDevice::seat_capabilities checks the pointer before
creating this object, it must be initially null so that the device
will be registered.

Change-Id: Ia15b407b1636eab41c04f30bb631306095e5aefb
Reviewed-by: Laszlo Agocs <laszlo.agocs@digia.com>

src/plugins/platforms/wayland/qwaylandinputdevice.cpp

@@ -232,6 +232,7 @@ QWaylandInputDevice::QWaylandInputDevice(QWaylandDisplay *display, uint32_t id)
     , mTouch(0)
     , mTime(0)
     , mSerial(0)
+    , mTouchDevice(0)
 {
     if (mQDisplay->dndSelectionHandler()) {
         mDataDevice = mQDisplay->dndSelectionHandler()->getDataDevice(this);