The method availableOutputBuffer() calculates how many chunks are in
use.  This simplifies code which needs this information, and it can
run without knowing OutputBuffer internals.  The function knows how to
calculate this when begin>end; this might have been a bug in
decodeParent(), which does not.

git-svn-id: https://svn.musicpd.org/mpd/trunk@7250 09075e82-0dd4-0310-85a5-a0d7c8717e4f