Don't load curves becore clearing their command stack.

Don't load curves becore clearing their command stack.

Loading a curve (which would happen if it had been unloaded before
being summoned by current_curve_callback) executes its command stack.
Since 'clear curve command stack' is only trying to remove the stack,
that stack execution is a waste of time.  With the unloaded callback,
we can remove the command stack without ever loading the curve.