Transform词云, 当size参数一个为0/null/undefined, 另一个为负数时 陷入死循环

* **Link**:
* **Platform**:
* **Mini Showcase(like screenshots)**:

例子：
let ds = [
{name: "有餐厅", value: "91"},
{name: "有吃饭的地", value: "58"},
{name: "有停车场", value: "50"},
{name: "西单边", value: "37"},
{name: "位置很好", value: "23"},
{name: "强烈推荐", value: "21"}
];

let dv = new DataSet.View().source(ds);

dv.transform({
type: 'tag-cloud',
fields: 100,
size: [0, -100],
font: 'serif',
padding: 0,
spiral: 'archimedean',
fontSize(d) { return d.value / 5 },
timeInterval: Infinity,
});

程序将在data-set/src/util/tag-cloud.js l311-l320 陷入死循环