h264: initialize frame-mt context copies properly

libavcodec/h264.c

@@ -1125,6 +1125,8 @@ static int decode_init_thread_copy(AVCodecContext *avctx)
     memset(h->sps_buffers, 0, sizeof(h->sps_buffers));
     memset(h->pps_buffers, 0, sizeof(h->pps_buffers));
 
+    h->s.context_initialized = 0;
+
     return 0;
 }