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

expect(response).toMatch(
        '<img ' + ROOT_ATTRIBUTE_NAME + '="" ' +
          ID_ATTRIBUTE_NAME + '="[^"]+" ' +
          ReactMarkupChecksum.CHECKSUM_ATTR_NAME + '="[^"]+"/>'
      );
    });
    it('should generate simple markup for attribute with `>` symbol', function() {
      var response = ReactServerRendering.renderToString(
        <img data-attr=">" />
      );
      expect(response).toMatch(
        '<img data-attr="&gt;" ' + ROOT_ATTRIBUTE_NAME + '="" ' +
          ID_ATTRIBUTE_NAME + '="[^"]+" ' +
          ReactMarkupChecksum.CHECKSUM_ATTR_NAME + '="[^"]+"/>'
      );
    });
    it('should generate comment markup for component returns null', function() {
      var NullComponent = React.createClass({
        render: function() {
          return null;
        },
      });
      var response = ReactServerRendering.renderToString(<NullComponent />);
      expect(response).toBe('<!-- react-empty: 1 -->');
    });
    it('should not register event listeners', function() {
      var EventPluginHub = require('EventPluginHub');
      var cb = jest.fn();
      ReactServerRendering.renderToString(
        <span onClick={cb}>hello world</span>