Fix Issue 12996 - SList: linearRemove cannot remove root node

```d
auto s = SList!int(1, 2, 3, 4, 5);
auto r = s[];
auto r1 = s.linearRemove(r); // exception
auto r1 = s.linearRemove(r.take(5)); // this works
```

https://issues.dlang.org/show_bug.cgi?id=12996