This patch fixes bug #4403. Directives
* rtems_timer_fire_when()
* rtems_timer_server_fire_when()
* rtems_task_wake_when()
are documented to return RTEMS_INVALID_ADDRESS when their time-of-day
argument is NULL. But actually they return RTEMS_INVALID_CLOCK. To fix
the issue this patch changes _TOD_Validate() to return a
status code instead of just true/false.
Close#4403