Avoid planner crash/Assert failure with joins to unflattened subqueries.

examine_simple_variable supposed that any RTE_SUBQUERY rel it gets pointed
at must have been planned already.  However, this isn't a safe assumption
because we must do selectivity estimation while generating indexscan paths,
and that code might look at join clauses involving a rel that the loop in
set_base_rel_sizes() hasn't reached yet.  The simplest fix is to play dumb
in such a situation, that is give up trying to extract any stats for the
Var.  This could possibly be improved by making a separate pass over the
RTE list to plan each unflattened subquery before we start the main
planning work --- but that would be pretty invasive and it doesn't seem
worth it, for now at least.  (We couldn't just break set_base_rel_sizes()
into two loops: the prescan would need to handle all subquery rels in the
query, not only those in the current join subproblem.)

This bug was introduced in commit 1cb108efb0e60d87e4adec38e7636b6e8efbeb57,
although I think that subsequent changes may have exposed it more than it
was originally.  Per bug #7580 from Maxim Boguk.

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