Decoder: Fix initialization of ps_next_dpb during reference list creation

ps_dpb_mgr->ps_dpb_ht_head is a linked list with
ps_dpb_mgr->u1_num_lt_ref_bufs number of valid buffers.
So ps_next_dbp has to be initialized to ps_dpb_mgr->ps_dpb_ht_head
for each u1_lt_index loop iteration

Bug: 33387820
Change-Id: I2dfd962bad5640a829afc123adbd0709e563e621