If no iio buffer has been set up and poll is called return 0.
Without this check there will be a null pointer dereference when
calling poll on a iio driver without an iio buffer.

Cc: stable@vger.kernel.org
Signed-off-by: NStefan Windfeldt-Prytz <stefan.windfeldt@axis.com>
Signed-off-by: NJonathan Cameron <Jonathan.Cameron@huawei.com>