Prevent signed overflow in get_time_seed

Prevent signed overflow in get_time_seed

Casting time(2) return value to int and multiplying the result with
such a constant will definitely lead to a signed overflow by this day.

Since signed overflows are undefined behaviour in C, avoid this.

Casting to unsigned is more than enough since the upper bits of a
64 bit time_t value will be removed with the int conversion anyway.