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.