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

var lifecycle = [];
        var TestComponent = React.createClass({
          componentWillMount: function() {
            lifecycle.push('componentWillMount');
          },
          componentDidMount: function() {
            lifecycle.push('componentDidMount');
          },
          getInitialState: function() {
            lifecycle.push('getInitialState');
            return {name: 'TestComponent'};
          },
          render: function() {
            lifecycle.push('render');
            return <span>Component name: {this.state.name}</span>;
          },
          componentWillUpdate: function() {
            lifecycle.push('componentWillUpdate');