Error upon conditional declaration inside a template

This looks related to #2670. Consider the following example:

``` nim
template someTempl(someConst: bool) =
  when someConst:
    var a : int
  if true:
    when not someConst:
      var a : int
    a = 5

someTempl(true)
```

```
test.nim(9, 10) template/generic instantiation from here
test.nim(7, 5) Error: expression 'a' has no type (or is ambiguous)
```
