MetaWaylandPopup: Dismiss popup when grab is cancelled

Dismiss the popup when the grab is cancelled, so that if the grab is
ended for whatever reason (such as VT switching or the last pointer
being disconnected), it doesn't try to end the grab when it isn't
active.

This fixes a crash when VT switching back and forth while a popup grab
is active.

https://bugzilla.gnome.org/show_bug.cgi?id=771858