[timer] set timer parent nil after deleted

[hongzhidao]

Hi!

diff -r 411ed164f393 src/nxt_timer.c
--- a/src/nxt_timer.c Fri Oct 27 11:05:37 2017 +0300
+++ b/src/nxt_timer.c Fri Nov 24 03:08:03 2017 -0500
@@ -220,6 +220,7 @@
timer->time, timer->state);

             nxt_rbtree_delete(&timers->tree, &timer->node);

•            nxt_timer_in_tree_clear(timer);
         }
  

Thanks.