table->partition_info is unitialized when we open frm file only.
The fix is check share->partition_info_len which is set on
frm opening stage.


mysql-test/r/information_schema_part.result:
  test result
mysql-test/t/information_schema_part.test:
  test case
sql/sql_show.cc:
  table->partition_info is unitialized when we open frm file only.
  The fix is check share->partition_info_len which is set on
  frm opening stage.