Rollback attempted to dereference DB_ROLL_PTR=0, which cannot possibly
be a valid undo log pointer. A safe canonical value would be
roll_ptr_t(1) << ROLL_PTR_INSERT_FLAG_POS
which is what was chosen in MDEV-12288.

This bug was reproduced in 10.3 only. Potentially, the problem could
have been introduced by MDEV-11415, which suppresses undo logging for
ALGORITHM=COPY operations. In those operations, we should actually
have written the safe value of DB_ROLL_PTR instead of writing 0.
However, the test in commit 5421e3ae
demonstrates that access to the rebuilt table by earlier-started
transactions should actually have been refused with ER_TABLE_DEF_CHANGED.

btr_cur_ins_lock_and_undo(): When undo logging is disabled, use the
safe value of DB_ROLL_PTR.

btr_cur_optimistic_insert(): Validate the DB_TRX_ID,DB_ROLL_PTR before
inserting into a clustered index leaf page.

ins_node_t::sys_buf[]: Replaces row_id_buf and trx_id_buf and some
heap usage.

row_ins_alloc_sys_fields(): Initialize ins_node_t::sys_buf[].

trx_undo_page_report_modify(): Assert that the DB_ROLL_PTR is not 0.

trx_undo_get_undo_rec_low(): Assert that the roll_ptr is valid before
trying to dereference it.

dict_index_t::is_primary(): Check if the index is the primary key.