build: Add missing object file for Matroska demuxer

Fixes compilation with --disable-everything --disable-network --enable-demuxer=matroska

Signed-off-by: Carl Eugen Hoyos <cehoyos@ag.or.at>
ffmpeg: fix memleak and corruption of AVSubtitle with multiple outputs

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>