graph: clear prio root iter on extra loop

Signed-off-by: Jens Axboe <axboe@kernel.dk>
diff --git a/graph.c b/graph.c
index 9761a1a..84bc9f9 100644
--- a/graph.c
+++ b/graph.c
@@ -916,6 +916,7 @@
 	best_delta = UINT_MAX;
 	i = g->labels;
 	do {
+		INIT_PRIO_TREE_ITER(&iter);
 		prio_tree_iter_init(&iter, &i->prio_tree, y, y);
 
 		n = prio_tree_next(&iter);