diag inconsistent behavior

Hi,
This comes up in both Julia 0.2.1 and 0.3.0-prerelease+3544. When I do something like

```
A = [1 2; 3 4]
diag(A, 2)
```

I get a bounds error, as expected. But if i do

```
diag(A, -2)
```

I get a 0 element array. Is this expected behavior of diag? Shouldn't both be giving bounds errors?

Thanks!
