added outputBufferChunkData()

Hiding OutputBuffer internals, again.  We get an extra assertion in
return.

git-svn-id: https://svn.musicpd.org/mpd/trunk@7267 09075e82-0dd4-0310-85a5-a0d7c8717e4f

src/outputBuffer.h

@@ -73,6 +73,8 @@ unsigned availableOutputBuffer(const OutputBuffer * cb);
  */
 int outputBufferAbsolute(const OutputBuffer * cb, unsigned relative);
 
+char * outputBufferChunkData(const OutputBuffer * cb, unsigned i);
+
 /* we send inStream for buffering the inputStream while waiting to
    send the next chunk */
 int sendDataToOutputBuffer(OutputBuffer * cb,