[Timer] Initialize the last tmrCtrls.

While initialize timer module, tmrCtrls is constructed to be a free
list. However, the last tmrCtrls is never initialized, which is
allocated from malloc(). Since the code relies on tmrCtrls->next
to tell if there's any free tmrCtrls left, the last tmrCtrls should
be properly linked to NULL.

src/util/src/ttimer.c

@@ -501,6 +501,7 @@ static void taosTmrModuleInit(void) {
     tmr_ctrl_t* ctrl = tmrCtrls + i;
     ctrl->next = ctrl + 1;
   }
+  (tmrCtrls + taosMaxTmrCtrl - 1)->next = NULL;
   unusedTmrCtrl = tmrCtrls;
 
   pthread_mutex_init(&tmrCtrlMutex, NULL);