Check for switched site context in CMB_Meta_Box::save()

[dnaber-de]

The method CMB_Meta_Box::save() assumes it runs in a site context it was initialized in. That means if any other component runs a switch_to_blog() and triggers the save_post action, your plugin writes data to a wrong object (in a wrong site). This already leads to concrete issues.

However, this is easy to solve by just adding the following guard to your save_post handler:

if ( is_multisite() && ms_is_switched() )
    return;

If appreciated I would provide a pull request.

[mikeselander]

This sounds reasonable to me, any thoughts @mattheu?