This bug was introduced in MySQL 5.6.8 with WL#6255.
When an error occurs while rebuilding a table that only has a
hidden GEN_CLUST_INDEX inside InnoDB, ha_alter_info->key_info_buffer
would be invalid and should not be dereferenced.

get_error_key_name(): Get the name of an erroneous key.
Avoid dereferencing ha_alter_info->key_info_buffer when no keys
exist in the SQL layer.

ha_innobase::inplace_alter_table(),
ha_innobase::commit_try_rebuild(): Invoke get_error_key_name()
for reporting ER_INNODB_ONLINE_LOG_TOO_BIG or ER_INDEX_CORRUPT.

RB: 13834
Reviewed-by: Jimmy Yang <jimmy.yang@oracle.com>