Fixed double free issue in events

Server crashed during shutdown with:
"corrupted double-linked list"
when running mysql_upgrade multiple times against the server.

Reason was that db_repostitory could be freed twice.

sql/events.cc

@@ -666,6 +666,7 @@ Events::drop_schema_events(THD *thd, const char *db)
     {
       db_repository->drop_schema_events(thd, &db_lex);
       delete db_repository;
+      db_repository= 0;
     }
   }
   DBUG_VOID_RETURN;