Negative Interpolation Variable Error

``` SASS
$variable: 1;

$interpolation: #{$variable}px;

div {
  top: -$interpolation;
}
```

Expected (3.3.5 and 3.2.19) output:

``` SASS
div {
  top: -1px;
}
```

libsass output:

``` SASS
div {
  top: -$interpolation; }
```


