Fixes #870 Not any last poster name in forumdisplay after splitting a/some post(s) (master)

Fixes #870 Not any last poster name in forumdisplay after splitting
a/some post(s)
Master branch (1.6)
How come there is no `update_thread_counters($tid, $counters);` and `update_last_post($post['tid']);` like in 1.8?