Fix storing -1 as node number

src/core/perfmodel/perfmodel.c

@@ -342,7 +342,7 @@ double starpu_task_expected_data_transfer_time(unsigned memory_node, struct star
 	{
 		starpu_data_handle_t handle = STARPU_TASK_GET_HANDLE(task, buffer);
 		enum starpu_data_access_mode mode = STARPU_TASK_GET_MODE(task, buffer);
-		unsigned node = -1;
+		int node = -1;
 		if (task->cl->specific_nodes)
 			node = STARPU_CODELET_GET_NODE(task->cl, buffer);
 		if (node == -1)