referenced_key_name field can be uninitialized in the case when
referenced table is dropped.
Added codition which allows to handle this situation.


mysql-test/r/information_schema_inno.result:
  test result
mysql-test/t/information_schema_inno.test:
  test result
sql/sql_show.cc:
  referenced_key_name field can be uninitialized in the case when
  referenced table is dropped.
  Added codition which allows to handle this situation.