queries if query was killed 
 
Since we rely on thd->is_error() to decide whether we should 
COMMIT or ROLLBACK after a query execution, check the query 
'killed' state and throw an error before calling 
ha_autocommit_or_rollback(), not after. 
 
The patch was tested manually. For reliable results, the test 
case would have to KILL QUERY while a DELETE/UPDATE query in 
another thread is still running. I don't see a way to achieve 
this kind of synchronization in our test suite (no debug_sync 
in 5.1).