block/nullb: fix NULL dereference

Dan reported this:

The patch 2984c868: "nullb: factor disk parameters" from Aug 14,
2017, leads to the following Smatch complaint:

drivers/block/null_blk.c:1759 null_init_tag_set()
	 error: we previously assumed 'nullb' could be null (see line
1750)

  1755		set->cmd_size	= sizeof(struct nullb_cmd);
  1756		set->flags = BLK_MQ_F_SHOULD_MERGE;
  1757		set->driver_data = NULL;
  1758
  1759		if (nullb->dev->blocking)
                    ^^^^^^^^^^^^^^^^^^^^
And an unchecked dereference.

nullb could be NULL here.
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Shaohua Li <shli@fb.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/block/null_blk.c

@@ -1756,7 +1756,7 @@ static int null_init_tag_set(struct nullb *nullb, struct blk_mq_tag_set *set)
 	set->flags = BLK_MQ_F_SHOULD_MERGE;
 	set->driver_data = NULL;
 
-	if (nullb->dev->blocking)
+	if ((nullb && nullb->dev->blocking) || g_blocking)
 		set->flags |= BLK_MQ_F_BLOCKING;
 
 	return blk_mq_alloc_tag_set(set);