Change history in the affected code:

- Since 10.4.8 (MDEV-20397 and MDEV-23311), functions ROUND(), CEILING(),
  FLOOR() return a TIME value for a TIME input.
- Since 10.4.14 (MDEV-23525), MIN() and MAX() calculate a result for a TIME
  input using val_native() rather than val_str().

Problem:

The patch for MDEV-23525 did not take into account combinations like
MIN(ROUND(time)), MAX(FLOOR(time)), etc.

MIN() and MAX() with ROUND(time), CEILING(time), FLOOR(time) as an argument
call the method val_native() of the undelying classes Item_func_round and
Item_func_int_val. However these classes implemented the method val_native()
as DBUG_ASSERT(0).

Fix:

This patch adds a TIME-specific code inside:
- Item_func_round::val_native()
- Item_func_int_val::val_native()
still with DBUG_ASSERT(0) for all other data types,
as other data types do not call val_native() of these classes.

We'll need a more generic solition eventualy, e.g.
turn Item_func_round and Item_func_int_val into Item_handled_func.
However, this change would be too risky for 10.4 at this point.