iio: adc: fix a potential NULL pointer dereference

devm_iio_trigger_alloc may fail and return NULL. The fix returns
ENOMEM when it fails.
Signed-off-by: Kangjie Lu <kjlu@umn.edu>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/adc/mxs-lradc-adc.c

@@ -465,6 +465,8 @@ static int mxs_lradc_adc_trigger_init(struct iio_dev *iio)
 
 	trig = devm_iio_trigger_alloc(&iio->dev, "%s-dev%i", iio->name,
 				      iio->id);
+	if (!trig)
+		return -ENOMEM;
 
 	trig->dev.parent = adc->dev;
 	iio_trigger_set_drvdata(trig, iio);