Merge change from MySQL:

  ChangeSet@1.2563, 2008-03-18 19:42:04+04:00, gluh@mysql.com +1 -0
    Bug#35406 5.1-opt crashes on select from I_S.REFERENTIAL_CONSTRAINTS
    added intialization of f_key_info.referenced_key_name for the case when
    referenced table is dropped