iio:kfifo_buf improve error handling in read_first_n.

These two elements were originally in the patch
iio:kfifo_buf  Take advantage of the fixed record size used in IIO
but Lars-Peter Clausen pointed out they should not have been
so here they are.
Signed-off-by: Jonathan Cameron <jic23@kernel.org>
Tested-by: Lars-Peter Clausen <lars@metafoo.de>

drivers/iio/kfifo_buf.c

@@ -108,10 +108,12 @@ static int iio_read_first_n_kfifo(struct iio_buffer *r,
 	int ret, copied;
 	struct iio_kfifo *kf = iio_to_kfifo(r);
 
-	if (n < r->bytes_per_datum)
+	if (n < r->bytes_per_datum || r->bytes_per_datum == 0)
 		return -EINVAL;
 
 	ret = kfifo_to_user(&kf->kf, buf, n, &copied);
+	if (ret < 0)
+		return ret;
 
 	return copied;
 }