[GtkClutterStandin] destroy the standin if the GCStandinBin is destroyed
[GtkClutterStandin] destroy the standin if the GCStandinBin is destroyed

When destroying a window, the GtkClutterStandinBin will be destroyed before
the GtkClutterStandin usually, leaving a dangling pointer to the bin in
GtkClutterStandinPrivate and generating a warning. Watch for the destruction
of the bin, and when it happens, set the pointer to NULL and destroy the
standin.

GNOME/clutter-gtk,Distrotech/clutter-gtk,Distrotech/clutter-gtk,Distrotech/clutter-gtk,GNOME/clutter-gtk