Segmentation fault using opt bool in if

```
var 0..1: x;
var bool: y = if x = 1 then true elseif false then <> else true endif = true;
```
```
$ minizinc -c f.mzn
MiniZinc error: Memory violation detected (segmentation fault).
This is a bug. Please file a bug report using the MiniZinc bug tracker.
Aborted
```
This happens on develop (30713e7). It works in 2.5.3.