branches/zip: When writing an externally stored column to the undo log, set the "external storage" flag.  When parsing the undo log, do not misinterpret a SQL NULL column for externally stored.
branches/zip: When writing an externally stored column to the undo log,
set the "external storage" flag.  When parsing the undo log, do not
misinterpret a SQL NULL column for externally stored.

These bugs were spotted by Heikki and Sunny.

trx_undo_page_report_modify(): Set the UNIV_EXTERN_STORAGE_FIELD flag
when needed.

trx_undo_rec_get_partial_row(): Check for len == UNIV_SQL_NULL.


ollie314/server,flynn1973/mariadb-aix,ollie314/server,flynn1973/mariadb-aix,natsys/mariadb_10.2,davidl-zend/zenddbi,flynn1973/mariadb-aix,slanterns/server,natsys/mariadb_10.2,davidl-zend/zenddbi,ollie314/server,flynn1973/mariadb-aix,davidl-zend/zenddbi,ollie314/server,flynn1973/mariadb-aix,natsys/mariadb_10.2,ollie314/server,flynn1973/mariadb-aix,ollie314/server,davidl-zend/zenddbi,flynn1973/mariadb-aix,flynn1973/mariadb-aix,flynn1973/mariadb-aix,natsys/mariadb_10.2,ollie314/server,davidl-zend/zenddbi,davidl-zend/zenddbi,davidl-zend/zenddbi,natsys/mariadb_10.2,natsys/mariadb_10.2,davidl-zend/zenddbi,natsys/mariadb_10.2,natsys/mariadb_10.2,natsys/mariadb_10.2,ollie314/server,natsys/mariadb_10.2,flynn1973/mariadb-aix,davidl-zend/zenddbi,ollie314/server,davidl-zend/zenddbi,natsys/mariadb_10.2,flynn1973/mariadb-aix,ollie314/server,ollie314/server,davidl-zend/zenddbi