mtd: nandsim: fix free of NULL pointer

If allocating ns->nand_pages_slab fails, do not try to destroy it when
cleaning up nandsim resources.
Signed-off-by: Sheng Yong <shengyong1@huawei.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>

drivers/mtd/nand/nandsim.c

@@ -649,6 +649,7 @@ static void free_device(struct nandsim *ns)
 				kmem_cache_free(ns->nand_pages_slab,
 						ns->pages[i].byte);
 		}
+		if (ns->nand_pages_slab)
 			kmem_cache_destroy(ns->nand_pages_slab);
 		vfree(ns->pages);
 	}