Wrong estimate of affected BLOB columns in update of PRIMARY KEY

MySQL 5.7.30 included the following bug fix:
Bug#30437378 : ASSERTION N_EXT == DTUPLE->GET_N_EXT() AT REC_GET_CONVERTED_SIZE() IN REM0REC.IC
It appears to be a follow-up to the MySQL 5.7.29 bug fix which we reported as MDEV-21511:
Bug#30342846 : N_EXT CALCULATION IS NOT CORRECT WHICH CAUSES WRONG RECORD SIZE CALCULATION
This refines an earlier fix in MySQL 5.7.5. I verified that the changed code is covered by the test innodb.blob-crash.

No test case was included for either the MySQL 5.7.29 or 5.7.30 code changes.

Only a part of the change applies to MariaDB. MDEV-11487 removed the function row_update_for_mysql_using_cursor(), which was unused in MariaDB Server.