lib/timeutils.c:strxxx_iso: test conversion errors

lib/timeutils.c:strxxx_iso: test conversion errors

Test for libc time conversion errors in ISO time format functions.

hwclock --utc --noadjfile --predict --date '67768034678846520 seconds'
Segmentation fault

Patched:
hwclock --utc --noadjfile --predict --date '67768034678846520 seconds'
hwclock: time 67768036191695381 is out of range.

Comparable to date(1):
date --date '67768034678846520 seconds'
date: time 67768036191695384 is out of range

Signed-off-by: J William Piggott <elseifthen@gmx.com>