bcachefs: Fix a null ptr deref in btree_iter_traverse_one()

When traversing nodes and we've reached the end of the btree, the
current btree node will be NULL.
Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/btree_iter.c

@@ -1148,7 +1148,8 @@ static int btree_iter_traverse_one(struct btree_iter *iter)
 	iter->uptodate = BTREE_ITER_NEED_PEEK;
 
 	bch2_btree_trans_verify_locks(iter->trans);
-	__bch2_btree_iter_verify(iter, iter->l[iter->level].b);
+	if (btree_iter_node(iter, iter->level))
+		__bch2_btree_iter_verify(iter, iter->l[iter->level].b);
 	return 0;
 }