False "allow.cartesian" error for a[!b] not-join

The following code:

``` R
a = data.table(v=c(1, 1), key="v")
b = data.table(v=c(1, 1), key="v")
a[!b]
```

raises error with wording "Join results in 4 rows; more than 2 = max(nrow(x),nrow(i))." Actually, not-join cannot produce more rows than nrow(x). This test can be safely removed.
