h264: fix null pointer dereference and assertion failure

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/h264.c

@@ -1727,7 +1727,7 @@ static int decode_update_thread_context(AVCodecContext *dst,
     h->data_partitioning    = h1->data_partitioning;
     h->low_delay            = h1->low_delay;
 
-    for (i = 0; i < MAX_PICTURE_COUNT; i++) {
+    for (i = 0; h->DPB && i < MAX_PICTURE_COUNT; i++) {
         h->DPB[i].period_since_free ++;
         unref_picture(h, &h->DPB[i]);
         if (h1->DPB[i].f.data[0] &&
@@ -1737,7 +1737,7 @@ static int decode_update_thread_context(AVCodecContext *dst,
 
     h->cur_pic_ptr     = REBASE_PICTURE(h1->cur_pic_ptr, h, h1);
     unref_picture(h, &h->cur_pic);
-    if ((ret = ref_picture(h, &h->cur_pic, &h1->cur_pic)) < 0)
+    if (h1->cur_pic.f.buf[0] && (ret = ref_picture(h, &h->cur_pic, &h1->cur_pic)) < 0)
         return ret;
 
     h->workaround_bugs = h1->workaround_bugs;