In e068d62a I added code that zeros the remainder of the output buffer
if there are not enough input frames available. I have now learned
that we can signal to the caller of the render callback how much data
is in the output buffers. In practice, the input buffer is so large
that it does not matter so much how we handle input buffer underruns,
but I suppose that saying 'there is no data' is better than 'here is
some silence for you'.

Fixes build failure when Vorbis is disabled, but FLAC is enabled
(HAVE_XIPH is true, but HAVE_OGG is false).

release v0.19.17

This is required if a stream ands without another chained FLAC file.

It's always 0.

If the duration is unknown, pass SignedSongTime::Negative(), as
documented for decoder_initialized().

This is obsolete because it has been moved to the MPD core.

Referencing the attribute "partition" is illegal after Close(),
because Close() deletes "this".

Fixes use-after-free bug (https://bugs.musicpd.org/view.php?id=4548).

The "seeking" flag is not set for the initial seek, and so
decoder_read() could be canceled when another SEEK was emitted during
initial seek.

This fixes several seek problems, for example the one reported for the
FLAC decoder plugin:

 https://bugs.musicpd.org/view.php?id=4552

libFLAC API documentation suggests that FLAC__stream_decoder_flush()
should be called to recover from FLAC__STREAM_DECODER_SEEK_ERROR.

Stop after all fatal errors.  This fixes assertion failures in
libFLAC.

No change for regular PCM, but DSD uses 0x69 now.