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

componentWillUnmount: function() {
            lifecycle.push('componentWillUnmount');
          },
        });
        var response = ReactServerRendering.renderToStaticMarkup(
          <TestComponent />
        );
        expect(response).toBe('<span>Component name: TestComponent</span>');
        expect(lifecycle).toEqual(
          ['getInitialState', 'componentWillMount', 'render']
        );
      }
      runTest();
      
      ExecutionEnvironment.canUseDOM = true;
      runTest();
    });
    it('should throw with silly args', function() {
      expect(
        ReactServerRendering.renderToStaticMarkup.bind(
          ReactServerRendering,
          'not a component'
        )
      ).toThrowError(
        'renderToStaticMarkup(): You must pass a valid ReactElement.'
      );
    });
    it('allows setState in componentWillMount without using DOM', function() {
      var Component = React.createClass({
        componentWillMount: function() {