Correctly write single ByteBuf with memoryAddress

Motivation:

While optimize gathering writes I introduced a bug when writing single ByteBuf that have a memoryAddress. This regression was introduced by 88bd6e7a9300073707f305409fa6481f1eeb2077.

Modifications:

Correctly use the writerIndex as argument when call Native.writeAddress(...)

Result:

No more corruption while write single buffers.