row_log_table_apply_ops runtime error: applying non-zero offset 1048576 to null pointer

In Debug build only (obvious by error location being in debug defines) the following UBSAN error occurred.

built with MSAN container per https://mariadb.com/kb/en/compile-and-using-mariadb-with-sanitizers-asan-ubsan-tsan-msan/

rpl.rpl_start_alter_5 'row'              w3 [ fail ]  Found warnings/errors in server log file!

        Test ended at 2025-06-18 04:39:11

line

/source/storage/innobase/row/row0log.cc:2695:6: runtime error: applying non-zero offset 1048576 to null pointer

Also tests: rpl.rpl_start_alter_3 rpl.rpl_start_alter_4 rpl.rpl_start_alter_5 rpl.rpl_start_alter_6 rpl.rpl_start_alter_1 rpl.rpl_start_alter_2 rpl.rpl_start_alter_mysqlbinlog_1

storage/innobase/row/row0log.cc:2693-2695

#ifdef UNIV_DEBUG

	if (next_mrec_end == index->online_log->head.block

	    + srv_sort_buf_size) {

So the index->online_log->head.block is a nullptr. I