The issue was that JOIN::rollup_write_data() used
JOIN::tmp_table_param::[start_]recinfo, which had uninitialized data.

These fields have uninitialized data, because JOIN::tmp_table_param
currently only stores some grouping-related data fields.  The data about
the work (temporary) tables themselves is stored in
join->join_tab[...].tmp_table_param.

The fix is to make JOIN::rollup_write_data follow this convention
and look at the right TMP_TABLE_PARAM object