Constant loop unroll creates invalid position

When compiler does loop unroll here:

```haxe
class Main {
static function main() {
var arr = [[]];
for( i in 0...5 )
if( i <= arr.length )
arr[i].push(i);
}
}
```

It produces invalid position:

```
haxe -hl main.hl -main Main && hl main.hl
Uncaught exception: Null access
Called from $Main.main(Main.hx:4) <-- should be line 6 !
Called from fun$331(?:1)
```