bcachefs: Fix an error path

It's possible to get -EIO in __btree_iter_traverse_all() after looping,
with orig_iter NULL.
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/btree_iter.c

@@ -1010,8 +1010,11 @@ static int __btree_iter_traverse_all(struct btree_trans *trans,
 
 	if (unlikely(ret == -EIO)) {
 		trans->error = true;
-		orig_iter->flags |= BTREE_ITER_ERROR;
-		orig_iter->l[orig_iter->level].b = BTREE_ITER_NO_NODE_ERROR;
+		if (orig_iter) {
+			orig_iter->flags |= BTREE_ITER_ERROR;
+			orig_iter->l[orig_iter->level].b =
+				BTREE_ITER_NO_NODE_ERROR;
+		}
 		goto out;
 	}