The problem: in an uncorrelated subquery, JOIN execution code may make 
irreversible cleanups after it has been executed (because the subquery 
won't be executed again). In particular, JTBM nests and their injected 
IN-equalities will be invalidated (the materialized table will be dropped
and TABLE structure freed).
Solution: don't walk into Item_subselect if represents an uncorrelated 
subselect that has already been executed. The idea is that the subselect
will not be executed again (calling Item_subselect_xxx::val_int() will fetch
the cached value), so it should not matter what is inside the subselect.