currentChunk is a global variable, which renders the whole output
buffer code non-reentrant.  Although this is not a real problem since
there is only one global output buffer currently, we should move it to
the OutputBuffer struct.

git-svn-id: https://svn.musicpd.org/mpd/trunk@7284 09075e82-0dd4-0310-85a5-a0d7c8717e4f