row_ins_clust_index_entry_low(): Do not attempt to read an AUTO_INCREMENT
column value from a metadata record, because it does not make any sense.
Moreover, the field offset would be off by one in case the AUTO_INCREMENT
column is not part of the PRIMARY KEY, because the MDEV-15562 metadata
record would contain an extra field at index->first_user_field().

On MariaDB Server 10.3 after MDEV-11369, we would unnecessarily read
a dummy AUTO_INCREMENT value from the metadata record, but that value
would always be written as NULL or 0, so there is no problem.