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.

src/core/ngx_times.c

@@ -211,6 +211,10 @@ ngx_time_sigsafe_update(void)
         slot++;
     }
 
+    tp = &cached_time[slot];
+
+    tp->sec = 0;
+
     ngx_gmtime(sec + cached_gmtoff * 60, &tm);
 
     p = &cached_err_log_time[slot][0];