message: read vtable size through ReadScalar()
diff: @@ -1364,7 +1364,7 @@ class FlatBufferBuilder {
  it += sizeof(uoffset_t)) {
  auto vt_offset_ptr = reinterpret_cast<uoffset_t *>(it);
  auto vt2 = reinterpret_cast<voffset_t *>(buf_.data_at(*vt_offset_ptr));
  - auto vt2_size = *vt2;
  + auto vt2_size = ReadScalar<voffset_t>(vt2);
  if (vt1_size != vt2_size || 0 != memcmp(vt2, vt1, vt1_size)) continue;
  vt_use = *vt_offset_ptr;
  buf_.pop(GetSize() - vtableoffsetloc);
  