UBI: fix kmem_cache_free on error patch

'kmem_cache_free()' oopeses if NULL is passed, and there is
one error-path place where UBI may call it with NULL object.
This problem was pointed to by Adrian Hunter.

Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
diff --git a/drivers/mtd/ubi/wl.c b/drivers/mtd/ubi/wl.c
index f25ae29..acb5520 100644
--- a/drivers/mtd/ubi/wl.c
+++ b/drivers/mtd/ubi/wl.c
@@ -826,7 +826,8 @@
 	err = schedule_erase(ubi, e1, 0);
 	if (err) {
 		kmem_cache_free(ubi_wl_entry_slab, e1);
-		kmem_cache_free(ubi_wl_entry_slab, e2);
+		if (e2)
+			kmem_cache_free(ubi_wl_entry_slab, e2);
 		goto out_ro;
 	}