Crosshair plugin runtime error on moving cursor out fix

[n-gist]

It is difficult to provide an example to test as the problem was discovered when using electron + react desktop application, but here is description and a screenshot of the error.

Having several graphs synchronized using synchronize plugin and having crosshair plugin used on them, doing these steps causes the error:

1. Zooming in one of the synced graphs
2. Moving cursor in and out of application bounds right after that (without clicking anywhere after zooming in)

Checking of selPoints_.length !== 0 fixed it