[media] lirc: fix null dereference for tx-only devices

tx-only RC devices do not have a receive buffer.
Signed-off-by: Sean Young <sean@mess.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>

drivers/media/rc/lirc_dev.c

@@ -468,7 +468,7 @@ int lirc_dev_fop_open(struct inode *inode, struct file *file)
 		if (retval) {
 			module_put(cdev->owner);
 			ir->open--;
-		} else {
+		} else if (ir->buf) {
 			lirc_buffer_clear(ir->buf);
 		}
 		if (ir->task)