math.Long#toString: Fix wrap around of intermediate int with radix 36

As of https://github.com/dcodeIO/Long.js/issues/7

```js
goog.math.Long.fromString("zzzzzz", 36).toString(36) == "-z141z5";
```

Which seems to be caused by an intermediate 32 bit integer wrap around in Long#toString. Fix and (basic) test attached.