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This is obsolete because it has been moved to the MPD core.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