iio: buffer: Return error if no callback is given

Return error in case no callback is provided to
`iio_channel_get_all_cb()`. There's no point in setting up a buffer-cb
if no callback is provided.
Signed-off-by: Nuno Sá <nuno.sa@analog.com>
Reviewed-by: Olivier Moysan <olivier.moysan@st.com>
Link: https://lore.kernel.org/r/20201121161457.957-3-nuno.sa@analog.comSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/buffer/industrialio-buffer-cb.c

@@ -54,6 +54,11 @@ struct iio_cb_buffer *iio_channel_get_all_cb(struct device *dev,
 	struct iio_cb_buffer *cb_buff;
 	struct iio_channel *chan;
 
+	if (!cb) {
+		dev_err(dev, "Invalid arguments: A callback must be provided!\n");
+		return ERR_PTR(-EINVAL);
+	}
+
 	cb_buff = kzalloc(sizeof(*cb_buff), GFP_KERNEL);
 	if (cb_buff == NULL)
 		return ERR_PTR(-ENOMEM);