MDEV-29149 Assertion `!is_valid_datetime() || fraction_remainder(((item->decimals) < (6) ? (item->decimals) : (6))) == 0' failed in Datetime_truncation_not_needed::Datetime_truncation_not_needed

TIME-alike string and numeric arguments to TIMEDIFF()
can get additional fractional seconds during the supported
TIME range adjustment in get_time().

For example, during TIMEDIFF('839:00:00','00:00:00') evaluation
in Item_func_timediff::get_date(), the call for args[0]->get_time()
returns MYSQL_TIME '838:59:59.999999'.

Item_func_timediff::get_date() did not handle these extra digits
and returned a MYSQL_TIME result with fractional digits outside
of Item_func_timediff::decimals. This mismatch could further be
caught by a DBUG_ASSERT() in various other pieces of the code,
leading to a crash.

Fix:

In case if get_time() returned MYSQL_TIMESTAMP_TIME,
let's truncate all extra digits using my_time_trunc(&l_time,decimals).
This guarantees that the rest of the code returns a MYSQL_TIME
with second_part not conflicting with Item_func_timediff::decimals.