Calling `setState` in `componentWillMount` of a component causes the error

```
TypeError: Cannot read property '_currentElement' of null
    at ReactCompositeComponentMixin._updateRenderedComponent
```

when rendering server-side with `ReactDOMServer.renderToString` and preceded by a component that renders content using `ReactDOMServer.renderToString` or `ReactDOMServer.renderToStaticMarkup`.

Causes TypeError with 0.14.8 and 15.0.0-rc.2:
https://github.com/PKJedi/react-dom-server-test/blob/master/test.js

Works with 0.13:
https://github.com/PKJedi/react-dom-server-test/blob/master/0.13/test.js

spyOn(console, 'error');
      instance = ReactDOM.render(<TestComponent name="y" />, element);
      expect(mountCount).toEqual(4);
      expect(console.error.calls.count()).toBe(1);
      expect(element.innerHTML.length > 0).toBe(true);
      expect(element.innerHTML).not.toEqual(lastMarkup);
      
      expect(numClicks).toEqual(1);
      ReactTestUtils.Simulate.click(ReactDOM.findDOMNode(instance.refs.span));
      expect(numClicks).toEqual(2);
    });
    it('should throw with silly args', function() {
      expect(
        ReactServerRendering.renderToString.bind(
          ReactServerRendering,
          'not a component'
        )
      ).toThrowError(
        'renderToString(): You must pass a valid ReactElement.'
      );
    });
  });
  describe('renderToStaticMarkup', function() {
    it('should not put checksum and React ID on components', function() {
      var NestedComponent = React.createClass({
        render: function() {
          return <div>inner text</div>;
        },
      });
      var TestComponent = React.createClass({