Problem:
Renaming a database to itself crashed server.
It hapenned because of wrong DBUG_ASSERT.
Fix: removing wrong DBUG_ASSERT. Now it reports
a correct error message "database alreadt exists".


mysql-test/r/renamedb.result:
  Adding test case
mysql-test/t/renamedb.test:
  Adding test case
sql/sql_db.cc:
  DBUG_ASSERT crashed server when renaming a database to itself.