fix unchecked length in stb_vorbis that could crash on corrupt/invalid files
static int start_packet(vorb *f)
{
   while (f->next_seg == -1) {
      if (!start_page(f)) return FALSE;
      if (f->page_flag & PAGEFLAG_continued_packet)
         return error(f, VORBIS_continued_packet_flag_invalid);
   }
   f->last_seg = FALSE;
   f->valid_bits = 0;
   f->packet_bytes = 0;
   f->bytes_in_seg = 0;
   return TRUE;
}

static int start_packet(vorb *f)
{
   while (f->next_seg == -1) {
      if (!start_page(f)) return FALSE;
      if (f->page_flag & PAGEFLAG_continued_packet)
         return error(f, VORBIS_continued_packet_flag_invalid);
   }
   f->last_seg = FALSE;
   f->valid_bits = 0;
   f->packet_bytes = 0;
   f->bytes_in_seg = 0;
   return TRUE;
}
