There are three issues that caused rpl_killed_ddl fails sporadically
in pb2:

 1) thd->clear_error() was not called before create Query event
if operation is executed successfully.
 2) DATABASE d2 might do exist because the statement to CREATE or
ALTER it was killed
 3) because of bug 43353, kill the query that do DROP FUNCTION or
    DROP PROCEDURE can result in SP not found

This patch fixed all above issues by:
 1) Called thd->clear_error() if the operation succeeded.
 2) Add IF EXISTS to the DROP DATABASE d2 statement
 3) Temporarily disabled testing DROP FUNCTION/PROCEDURE IF EXISTS.

mysql-test/t/rpl_killed_ddl.test:
  DATABASE d2 might not exists, add IF EXITS to the DROP statement
sql/sql_db.cc:
  Called thd->clear_error() if the operation succeeded