Fixed possible use of old cached times if runtime went backwards.

If ngx_time_sigsafe_update() updated only ngx_cached_err_log_time, and
then clock was adjusted backwards, the cached_time[slot].sec might
accidentally match current seconds on next ngx_time_update() call,
resulting in various cached times not being updated.

Fix is to clear the cached_time[slot].sec to explicitly mark cached times
are stale and need updating.