Consider a query with subquery in form t.key=(select ...). Suppose, the
parent query uses this equality for ref access.
It will attempt to evaluate the subquery in get_best_combination(),
right before the join->join_tab[...] array is filled.  The problem was
that subquery optimization will attempt to look at parent's join->join_tab
to check how many times subquery will be executed (and crash).

Fixed by not doing that when the subquery is constant (non-constant
subqueries are only be evaluated during join execution, so they are not
affected)