In the WhereClause object, do not assume that all TERM_VIRTUAL terms appear

In the WhereClause object, do not assume that all TERM_VIRTUAL terms appear
at the end of the list, because that is no longer true.  Instead, keep a
separate nBase count that is the size of the list excluding the tail of
virtual terms.  Use nBase instead of nTerm when scanning terms that are not
virtual.  Add assert()s to validate correctness of WhereClause.

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