BLE: nil pointer dereference on disconnect possible

[gen2thomas]

To initiate a retry after a failing "robot.Start()", an "robot.Stop()" could be useful to get rid of all devices, which are successfully connected until this point.

If the BLE device was not connected at this time, the disconnect fails because the "extDevice" is still not defined in btwrapper.go.

// Disconnect from the BLE device. This method is non-blocking and does not wait until the connection is fully gone.
func (btd *btDevice) disconnect() error {
	return btd.extDevice.Disconnect()
}

this needs to be changed to something like this:

// Disconnect from the BLE device. This method is non-blocking and does not wait until the connection is fully gone.
func (btd *btDevice) disconnect() error {
        if btd!=nil && btd.extDevice!= nil{
            return btd.extDevice.Disconnect()
        }
	return nil
}