iio: buffer: Fix NULL pointer deref caused by empty scan mask

iio_scan_mask_match() returns NULL if the passed in scan mask is empty. This
will happen if no channel has been selected and buffer is enabled.
iio_sw_buffer_preenable() will assign NULL to indio_dev->active_scan_mask in
this case. As a result iio_update_demux() will cause a NULL pointer deref,
because it expects active_scan_mask to be non-NULL.

Since it does not make much sense to start data capture if there is no data to
capture this patch updates the code to fail gracefully in iio_scan_mask_match()
instead of crashing the kernel.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Acked-by: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>