staging:iio:max1363: Don't free uninitialized variable

It is possible that on one of the error paths we are going to try to free
'rxbuf', even though it has not been allocated yet, which cause the following
warning:
	drivers/staging/iio/adc/max1363_ring.c: In function 'max1363_trigger_handler':
		drivers/staging/iio/adc/max1363_ring.c:87:7: warning: 'rxbuf' may be used
		uninitialized in this function

Reported-by: Fengguang Wu <wfg@linux.intel.com>
Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>

drivers/staging/iio/adc/max1363_ring.c

@@ -74,7 +74,7 @@ static irqreturn_t max1363_trigger_handler(int irq, void *p)
 	else
 		b_sent = i2c_master_recv(st->client, rxbuf, numvals);
 	if (b_sent < 0)
-		goto done;
+		goto done_free;
 
 	time_ns = iio_get_time_ns();
 
@@ -82,9 +82,10 @@ static irqreturn_t max1363_trigger_handler(int irq, void *p)
 		memcpy(rxbuf + d_size - sizeof(s64), &time_ns, sizeof(time_ns));
 	iio_push_to_buffer(indio_dev->buffer, rxbuf, time_ns);
 
+done_free:
+	kfree(rxbuf);
 done:
 	iio_trigger_notify_done(indio_dev->trig);
-	kfree(rxbuf);
 
 	return IRQ_HANDLED;
 }