alter event rename".

ALTER EVENT ... RENAME statement hasn't checked privileges
for the target database. It also caused server crashes when
target database was not specified explicitly and there was
no current database.

This fix adds missing privilege check and check for the case
when target database is not specified explicitly or implicitly.


mysql-test/r/events_bugs.result:
  update result
mysql-test/t/events_bugs.test:
  add test case for bug 18897 Events: unauthorized action possible with alter event
  rename:
  - test rename to db the user does not have access to
  - test rename when there is no selected db
sql/sql_parse.cc:
  Additional check for the situation when no db is selected.
  CREATE EVENT abc and ALTER EVENT db.abc RENAME TO xyz,
  and DROP EVENT abc
  won't work if there is no selected DB.