drbd: null dereference bug

epoch is always NULL here.
Signed-off-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: Philipp Reisner <philipp.reisner@linbit.com>

drivers/block/drbd/drbd_receiver.c

@@ -1224,7 +1224,7 @@ static int receive_Barrier(struct drbd_conf *mdev, struct p_header *h)
 	epoch = kmalloc(sizeof(struct drbd_epoch), GFP_NOIO);
 	if (!epoch) {
 		dev_warn(DEV, "Allocation of an epoch failed, slowing down\n");
-		issue_flush = !test_and_set_bit(DE_BARRIER_IN_NEXT_EPOCH_ISSUED, &epoch->flags);
+		issue_flush = !test_and_set_bit(DE_BARRIER_IN_NEXT_EPOCH_ISSUED, &mdev->current_epoch->flags);
 		drbd_wait_ee_list_empty(mdev, &mdev->active_ee);
 		if (issue_flush) {
 			rv = drbd_flush_after_epoch(mdev, mdev->current_epoch);