Error is thrown if drop is destroyed before hover timeout executes

[zmdavis]

1. Create a drop that opens on hover with a hoverOpenDelay
2. Hover over target, which sets a timeout
3. Destroy the drop (before timeout executes)
4. Timeout is not cleared, and when it executes a js error is thrown

The actual error thrown is Cannot read property 'parentNode' of null and is caused by https://github.com/HubSpot/drop/blob/master/src/js/drop.js#L374

If this.drop is null when open is called it should probably just return instead of trying to do things.

[streamside]

Looks like a duplicate of #139.

[zmdavis]

Indeed. Sorry about that, I only looked for existing open issues, not open PRs.