window: Return -1 if meta_window_get_monitor is called on an unmanaged window

As opposed to crashing. In this case, letting the caller deal with it is the best policy, since this is public API.

https://bugzilla.gnome.org/show_bug.cgi?id=788834

---

Reproposed the MP here with the patch by @sam1 as this is a quite occurring crash and we can track it better on gitlab.