avformat/libopenmpt: Query duration and metadata after selecting subsong

avformat/libopenmpt: Query duration and metadata after selecting subsong

Duration depends on the selected subsong and thus must be queried after
selecting the subsong. There is no compelling reason to query other
metadata earlier either.

Tested with libopenmpt version: 0.2.8760-beta27
Libopenmpt configure options: --without-ogg --without-vorbis
--without-vorbisfile --without-portaudio --without-portaudiocpp
--without-mpg123 --without-pulseaudio --without-sndfile --without-flac

Signed-off-by: Jörn Heusipp <osmanx@problemloesungsmaschine.de>
Signed-off-by: Josh de Kock <josh@itanimul.li>