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

render: function() {
          return <span><NestedComponent /></span>;
        },
      });
      var response = ReactServerRendering.renderToStaticMarkup(
        <TestComponent />
      );
      expect(response).toBe('<span><div>inner text</div></span>');
    });
    it('should not put checksum and React ID on text components', function() {
      var TestComponent = React.createClass({
        render: function() {
          return <span>{'hello'} {'world'}</span>;
        },
      });
      var response = ReactServerRendering.renderToStaticMarkup(
        <TestComponent />
      );
      expect(response).toBe('<span>hello world</span>');
    });
    it('should not register event listeners', function() {
      var EventPluginHub = require('EventPluginHub');
      var cb = jest.fn();
      ReactServerRendering.renderToString(
        <span onClick={cb}>hello world</span>
      );
      expect(EventPluginHub.__getListenerBank()).toEqual({});
    });
    it('should only execute certain lifecycle methods', function() {
      function runTest() {