Check for `cancelAnimationFrame` when we call it.

`stopCheckChange()` is called twice, and in [one](https://github.com/ginader/HTML5-placeholder-polyfill/blob/5805e4d11cd8790cf0af422fb487df5a61dc81e3/src/placeholder_polyfill.jquery.js#L56) instance, it isn't checking for `window.cancelAnimationFrame` first.