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.