date_add_interval(): signed overflow on huge YEAR/MONTH interval values

INTERVAL::year and INTERVAL::month are unsigned, but date_add_interval() (sql/sql_time.cc) casts them to long and does signed arithmetic before the range check:

case INTERVAL_YEAR:

  ltime->year+= sign * (long) interval.year;

...

case INTERVAL_MONTH:

  long period= (ltime->year*12 + sign * (long) interval.year*12 +

                ltime->month-1 + sign * (long) interval.month);

With values above LONG_MAX (or LONG_MAX/12 for years) the cast is implementation-defined and the multiplication/addition is signed overflow, which is undefined behaviour.

In practice the current builds still end in a clean error, so this is not a visible wrong result today; UBSAN should flag it:

select date_add('2000-01-01', interval 9223372036854775808 month),

       date_add('2000-01-01', interval 768614336404564651 year);

-- NULL, NULL, Warning 1441 Datetime function: datetime field overflow

create table t1 (x int) with system versioning

partition by system_time interval 9223372036854775808 month;

-- ERROR 1690 TIMESTAMP value is out of range in 'INTERVAL'

Expected: check the year/month magnitude (as is already done for days with MAX_DAY_NUMBER) before the arithmetic.

Present in 10.11 and later.

Found during review of MDEV-25529 (https://github.com/MariaDB/server/pull/5294); the code predates that change.