Buffer sizes should be size_t.  This is safe here, at least not
unsafer than without the patch.  I have no idea why audioBufferSize
and audioBufferPos were explicitly declared as signed integer.

git-svn-id: https://svn.musicpd.org/mpd/trunk@7296 09075e82-0dd4-0310-85a5-a0d7c8717e4f