subselect_single_select_engine::exec()

When a subquery doesn't need to be evaluated because
it returns only aggregate functions and these aggregates
can be calculated from the metadata about the table it
was not updating all the relevant members of the JOIN 
structure to reflect that this is a constant query.
This caused problems to the enclosing subquery 
('<> SOME' in the test case above) trying to read some
data about the tables.

Fixed by setting const_tables to the number of tables 
when the SELECT is optimized away.