Valgrind warnings were caused by comparing index values to an un-initialized field.


mysql-test/r/subselect.result:
  New test cases.
mysql-test/t/subselect.test:
  New test cases.
sql/opt_sum.cc:
  Add thd to opt_sum_query enabling it to test for errors.
  If we have a non-nullable index, we cannot use it to match null values,
  since set_null() will be ignored, and we might compare uninitialized data.
sql/sql_select.cc:
  Add thd to opt_sum_query, enabling it to test for errors.
sql/sql_select.h:
  Add thd to opt_sum_query, enabling it to test for errors.