Check if scrollbar exists before calling destroy function

[ghost]

In some cases componentWillUnmount is called without componentDidMount beforehand.

For example:

• Error in provide when componentWillUnmount called without componentDidMount FormidableLabs/freactal#40
• Cryptic error using webpack v2, webpack-dev-server v2, RHL v3 gaearon/react-hot-loader#342
• bug componentWillUnmount is called before componentDidMount  facebook/react#5719

To be a bit more resilient is such cases I've added this workaround which checks if scrollbar was instantiated before calling it's destroy method.

This doesn't solve the underlying bugs which cause this wrong lifecycle-method calling but it means there isn't an exception from scrollbar in such cases.

[idiotWu]

Thanks!