Opened by sox_open_memstream_write function，the buffer and buffersize will work incorrectly when seeking back。

For example， when opening buffer with sox_open_memstream_write to get a wav format output，wav handler's  stopwrite function will seek back to the beginning to rewrite wav header. In this situation，after closing ft，I assume that I can get whole data from buff and buffsize，but I can only get the wav header. 

I don't think it would be better to have seek back to the end when you need to rewrite data with a seek operation.