h264: copy nal_length_size in update_thread_context

It does not make sense to copy is_avc without copying this as well. This
patch should not change anything for now, but will have an effect in
later commits.

libavcodec/h264_slice.c

@@ -510,6 +510,7 @@ int ff_h264_update_thread_context(AVCodecContext *dst,
 
     // extradata/NAL handling
     h->is_avc = h1->is_avc;
+    h->nal_length_size = h1->nal_length_size;
 
     // SPS/PPS
     if ((ret = copy_parameter_set((void **)h->sps_buffers,