Fix crash when touching stale Automation Points

Fix crash when touching stale Automation Points

Undo/Redo allows to change a region's envelope without the envelope-list
changing (AutomationList::StateChanged is not emitted).

This also catches other operations where region-length can change while
the Envelope is visible. -- Normally when using the range or object tool
to modify a Region, the Envelope is not visible.

Without this change it was possible that the GUI shows automation
points that don't have a corresponding libardour representation:

e.g. select range, change tool to "draw", press "s" (split), undo,
click on any region-gain point.