The segiter code used objptridx because end is signalled by object_none,
and vobjptridx does not allow object_none. However, the compiler
produces better code if segment_object_range_t returns vobjptridx and
future cleanups are easier if iterating objects_in yields vobjptridx
objects. Add a special-case override of the normal checking rules, move
the required checks into segiter, and then let segiter break the rules
normally enforced by valptridx. Add a comment explaining that this
permits unsafe coding and should be done only with a clear understanding
of the responsibilities it brings.
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