values return too many records

WHERE clauses with "outer_value_list NOT IN subselect" were
handled incorrectly if the outer value list contained multiple 
items where at least one of these could be NULL. The first 
outer record with NULL value was handled correctly, but if a 
second record with NULL value existed, the optimizer would 
choose to reuse the result it got on the last execution of the 
subselect. This is incorrect if the outer value list has 
multiple items.
     
The fix is to make Item_in_optimizer::val_int (in 
item_cmpfunc.cc) reuse the result of the latest execution
for NULL values only if all values in the outer_value_list 
are NULL.