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>
diff --git a/drivers/mtd/nand/nandsim.c b/drivers/mtd/nand/nandsim.c
index 52c0c1a..6a74f62 100644
--- a/drivers/mtd/nand/nandsim.c
+++ b/drivers/mtd/nand/nandsim.c
@@ -649,7 +649,8 @@
 				kmem_cache_free(ns->nand_pages_slab,
 						ns->pages[i].byte);
 		}
-		kmem_cache_destroy(ns->nand_pages_slab);
+		if (ns->nand_pages_slab)
+			kmem_cache_destroy(ns->nand_pages_slab);
 		vfree(ns->pages);
 	}
 }