fix: ensure ref is present before using

[STRML]

It's possible for the ref to be undefined, especially when using async batching strategies.

For example:

TypeError: Cannot read property 'getBoundingClientRect' of null
at getSliderStart(node_modules/rc-slider/lib/createSlider.js:170:0)
at calcValueByPos(node_modules/rc-slider/lib/createSlider.js:196:0)
at onMove(node_modules/rc-slider/lib/Slider.js:111:0)
at callback(node_modules/rc-slider/lib/createSlider.js:120:0)
at batchedUpdates(ui/utils/ReactRAFBatchingStrategy.js:29:13)
at unstable_batchedUpdates(node_modules/react-dom/lib/ReactUpdates.js:97:0)
at call(node_modules/rc-util/lib/Dom/addEventListener.js:21:0)
at apply(node_modules/add-dom-event-listener/lib/index.js:17:0)
at apply(node_modules/raven-js/src/raven.js:278:0)

[coveralls]

Coverage decreased (-0.3%) to 65.116% when pulling 4530398 on STRML:check-ref into ea13b42 on react-component:master.

[benjycui]

Could you add a unit test for this in https://github.com/react-component/slider/blob/master/tests/common.test.js ? THX.

[STRML]

Updated - also noticed that these event handlers could remain in the DOM if the component is unmounted before mouseup/touchend. Added a componentWillUnmount and corresponding test.

[coveralls]

Coverage increased (+1.08%) to 66.474% when pulling 9aa17d6 on STRML:check-ref into ea13b42 on react-component:master.

[coveralls]

Coverage increased (+1.08%) to 66.474% when pulling 749a13a on STRML:check-ref into ea13b42 on react-component:master.

[benjycui]

6.0.2