avcodec/h264: Fix poc_lsb in open gop context

When no IDR nor mmco_reset is found, prev_poc_lsb is
undefined and shall not be assumed to be zero

libavcodec/h264_parse.c

@@ -287,6 +287,8 @@ int ff_h264_init_poc(int pic_field_poc[2], int *pic_poc,
 
     if (sps->poc_type == 0) {
         const int max_poc_lsb = 1 << sps->log2_max_poc_lsb;
+        if (pc->prev_poc_lsb < 0)
+            pc->prev_poc_lsb =  pc->poc_lsb;
 
         if (pc->poc_lsb < pc->prev_poc_lsb &&
             pc->prev_poc_lsb - pc->poc_lsb >= max_poc_lsb / 2)

libavcodec/h264dec.c

@@ -474,7 +474,7 @@ static void idr(H264Context *h)
     h->poc.prev_frame_num        =
     h->poc.prev_frame_num_offset = 0;
     h->poc.prev_poc_msb          = 1<<16;
-    h->poc.prev_poc_lsb          = 0;
+    h->poc.prev_poc_lsb          = -1;
     for (i = 0; i < MAX_DELAYED_PIC_COUNT; i++)
         h->last_pocs[i] = INT_MIN;
 }