init mTouchDevice to null
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 <00c05606d635b5d8e1450a8536e4e2809abe4e80@digia.com>

locusf/qtwayland-1,ntanibata/qtwayland,ntanibata/qtwayland,locusf/qtwayland-1,locusf/qtwayland-1,ntanibata/qtwayland,Tofee/qtwayland,locusf/qtwayland-1,Tofee/qtwayland,Tofee/qtwayland