Add assert() statements that fail when a shallow copy is accessed after the

Add assert() statements that fail when a shallow copy is accessed after the
original has been modified.  These assert() statements should detect the kinds
of subtle SCopy bugs such as caused the fault
in ticket [b351d95f9cd5ef17e9d9dbae18f].

FossilOrigin-Name: 8b8e1732e8410efd5f8e308c43abe521584e87a0