order(list_column) gives strange error

```
DT = data.table(
  V = list(71L, c(71L, 132L), c(71L, 132L, 155L), 97L, 155L)
)
DT[order(V)]
# Error: Column 2 is length 2 which differs from length of column 1 (1)
```

I had forgotten `V` is a list column (and not `character` which can be sorted), but it seems the command went further than expected.