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

ExecutionEnvironment.canUseDOM = true;
      runTest();
    });
    it('should have the correct mounting behavior', function() {
      
      ExecutionEnvironment.canUseDOM = true;
      var mountCount = 0;
      var numClicks = 0;
      var TestComponent = React.createClass({
        componentDidMount: function() {
          mountCount++;
        },
        click: function() {
          numClicks++;
        },
        render: function() {
          return (
            <span ref="span" onClick={this.click}>Name: {this.props.name}</span>
          );
        },
      });
      var element = document.createElement('div');
      ReactDOM.render(<TestComponent />, element);
      var lastMarkup = element.innerHTML;
      
      
      ReactDOM.render(<TestComponent name="x" />, element);
      expect(mountCount).toEqual(1);