Implicit return with case expression fails with 'var' return.

``` nim
type
  Foo = ref object
    x: seq[string]

proc get_x(x: Foo): var seq[string] =
  case true
  of true:
    x.x
  else:
    x.x

var foo = Foo(x: @[])
foo.get_x().add("asd")
```

Error:

```
Error: expression has no address
```

Works when explicitly using `return`.
