fix tree traversal when unpending

the continue statement continued the inner loop, in fact the while
(node->state >= ST_UNPENDING) was intended to be continued.

I could neither convince myself that this can not cause an infinite loop
nor could I convince myself that it does.

src/node.c

@@ -1017,12 +1017,14 @@ worklist_unpend(struct req *req, struct bytes_tree *tree,
 				next = VSTAILQ_NEXT(node, sibling);
 				if (next != NULL) {
 					node = next;
-					continue;
+					break;
 				}
 				node = node->parent;
 			}
 
-			AZ(node);
+			if (node != NULL)
+				continue;
+
 			VSLdbg(req, "bytes_unpend: reached root");
 			set_front(tree, tree->root, req->wrk);
 			return;