Fixed lp:828514 "Assertion `! is_set()' failed in Diagnostics_area::set_ok_status with derived table + subquery + concurrent DML"


sql/item_subselect.cc:
  Added check of error condtions (safety)
sql/sql_join_cache.cc:
  Added DBUG to some functions.
  Added error checking for calls to check_match(); This fixed the bug.
sql/sql_select.cc:
  Moved variable assignment to be close to where it's used (cleanup)