mysql-test/r/subselect.result:
  Fix for bug #18306: MySQL crashes and restarts using subquery
  test case
mysql-test/t/subselect.test:
  Fix for bug #18306: MySQL crashes and restarts using subquery
  test case
sql/opt_range.cc:
  Fix for bug #18306: MySQL crashes and restarts using subquery
  Restore thd->mem_root because
  during the cond->val_int() evaluation we can come across a subselect 
  item which may allocate memory on the thd->mem_root and assumes
  all the memory allocated has the same life span as the subselect
  item itself.