Co-authored-by: NShuduo Sang <sdsang@taosdata.com>

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.