MDEV-18625 ASAN unknown-crash in my_copy_fix_mb / ha_mroonga::storage_inplace_alter_table_add_column

disable inplace alter for adding stored generated columns.

This fixes mroonga/storage.column_generated_stored_add_column failures
in ASAN_OPTIONS="abort_on_error=1" runs

Also, add a test case that shows the bug without ASAN.