h264: check that DPB is allocated before accessing it in flush_dpb()

libavcodec/h264.c

@@ -2623,6 +2623,7 @@ static void flush_dpb(AVCodecContext *avctx)
 
     flush_change(h);
 
+    if (h->DPB)
         for (i = 0; i < MAX_PICTURE_COUNT; i++)
             unref_picture(h, &h->DPB[i]);
     h->cur_pic_ptr = NULL;