slave_sql thread calls thd->clear_error() to force error to be ignored,
though this method didn't clear thd->killed state, what causes
slave_sql thread to stop.

clear thd->killed state if we ignore an error


mysql-test/r/rpl_skip_error.result:
  Bug #28839 Errors in strict mode silently stop SQL thread if --slave-skip-errors exists.
  
  test result
mysql-test/t/rpl_skip_error.test:
  Bug #28839 Errors in strict mode silently stop SQL thread if --slave-skip-errors exists.
  
  test case
sql/log_event.cc:
  Bug #28839 Errors in strict mode silently stop SQL thread if --slave-skip-errors exists.
  
  clear thd->killed state if we ignore the error