The fix in MDEV-14022 was incomplete. We must adjust some more code
for the fact that SYS_INDEXES records may lack the column MERGE_THRESHOLD
that was "instantly" added in MySQL 5.7 and MariaDB 10.2.2.

dict_boot(): Hard-core SYS_INDEXES.MERGE_THRESHOLD as DEFAULT NULL.

dict_index_t::instant_field_value(), rec_offs_make_valid():
Tolerate the missing SYS_INDEXES.MERGE_THRESHOLD column.