timedated: extra overflow safety check when doing relative time changes

Ensure clients don't overflow usec_t when doing relative time changes.
This is mostly just paranoia and protection against accidents, after all
clients are already authenticated, and they can se the time to any
value they wish anyway, but better be safe than sorry.

https://bugs.launchpad.net/ubuntu/+source/systemd/+bug/1152187/comments/14