PR# 11468 Stepping too fast in debugger causes wrong items to be highlighted as changed

Stepping too fast in the debugger causes the wrong items to be highlighted as
changed (and also too many items, it appears).

To reproduce:  Compile sample graphical application.  Put breakpoint on
{EV_APPLICATION_IMP}.process_gdk_events.  Click Start.  Step slowly using F10.
Attribute `default_font_name_internal' never changes, which is not surprising.
Now click rapidly on F10 for 5-10 times, then stop clicking.  Keep repeating. Soon `default_font_name_internal'
is highlighted as though it had changed.  Careful examination reveals that it
*has* changed - but only the address has changed.  Somehow, stepping too quickly
causes the address to change sometimes, which then causes that row to be
highlighted in pink as changed.  Very confusing.