Prevent a crash in the situation where:

Prevent a crash in the situation where:
 1. session teardown starts
 2. editor responds by removing and destroying all TimeAxisViews
 3. session then asks all routes to drop refs
 4. the editor is called by each route's drop refs handler to remove the associated TimeAxisView
 5. that TimeAxisView has already been destroyed in (2)

git-svn-id: svn://localhost/ardour2/branches/3.0@6392 d708f5d6-7413-0410-9779-e7cbd77b26cf