The convert_constant_item() function converts constant items to ints on
prepare phase to optimize execution speed. In this case it tries to evaluate
subselect which contains a derived table and is contained in a derived table. 
All derived tables are filled only after all derived tables are prepared.
So evaluation of subselect with derived table at the prepare phase will
return a wrong result.

A new flag with_subselect is added to the Item class. It indicates that
expression which this item represents is a subselect or contains a subselect.
It is set to 0 by default. It is set to 1 in the Item_subselect constructor
for subselects.
For Item_func and Item_cond derived classes it is set after fixing any argument
in Item_func::fix_fields() and Item_cond::fix_fields accordingly.
The convert_constant_item() function now doesn't convert a constant item
if the with_subselect flag set in it.