Fix isc_time_add() overflow
The isc_time_add() could overflow when t.seconds + i.seconds == UINT_MAX and t.nanoseconds + i.nanoseconds >= NS_PER_S. Fix the overflow in isc_time_add(), and simplify the ISC_R_RANGE checks both in isc_time_add() and isc_time_sub().
Edited by Ondřej Surý