Verify that tv_sec value specified in settimeofday() and clock_settime()
(CLOCK_REALTIME case) system calls is non negative.
This commit hides a kernel panic in atrtc_settime() as the clock_ts_to_ct()
does not properly convert negative tv_sec.

ps. in my opinion clock_ts_to_ct() should be rewritten to properly handle
negative tv_sec values.

Differential Revision: https://reviews.freebsd.org/D4714
Reviewed by: kib

MFC after: 1 week