I started activating new program_state.saving and discovered that
saving of ball and chain could access freed memory. The change
for the former and fix for the latter are mixed together here (but
easily distinguishable).
The saving flag inhibits status updating and perm_invent updating,
also map updating that goes through flush_screen(). That should
fix the exception triggered after an impossible warning was issued
during a save operation. impossible() goes through pline() which
tries to bring the screen up to date before issuing a message.
During save, data for that update can be in an inconsistent state.
The code to save ball and/or chain when not on floor or in invent
(I think swallowed is the only expected case) was examining the
memory pointed to by uball and uchain even if saving the level had
just freed floor objects and saving invent had just freed carried
objects. So for the usual cases, stale pointer values for uball
and uchain would be present and checking their obj->where field
was not reliable.
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