Check for NULL in addTracks()

    for (size_t index = 0; index < tracks->GetTracksCount(); ++index) {

        const mkvparser::Track *track = tracks->GetTrackByIndex(index);

        if (!track) continue; // we can get back a NULL (it happens...)

        const char *const codecID = track->GetCodecId();

        LOGV("codec id = %s", codecID);