Conversion errors when constructing the condition for an
IN predicates were treated as if the affected column contains
NULL. If such a IN predicate is inside NOT we get wrong 
results.
Corrected the handling of conversion errors in an IN predicate 
that is resolved by unique_subquery (through 
subselect_uniquesubquery_engine).


mysql-test/r/subselect3.result:
  Bug#22855: test case
mysql-test/t/subselect3.test:
  Bug#22855: test case
sql/item_subselect.cc:
  Bug#22855: corrected the handling of conversion errors and
  NULL key values in IN predicate that is resolved by index
  lookup.