When a tree view is detroyed, its GailTreeView is destroyed as well but the
weak references are kept and thus point to a freed memory block. A
segfault may arise when the model is destroyed in turn and the weak-ref
mechanism tries to clear the pointers in freed memory.
This patch removes the weak references either when the GailTreeView is
being freed or when a new model is attached to the underlying GtkTreeView.
https://bugzilla.gnome.org/show_bug.cgi?id=694711
8b719abcbb