StackOverflow with sin

With PR https://github.com/JuliaIntervals/IntervalConstraintProgramming.jl/pull/212 we get the following rather unhelpful error message:

```jl
julia> S = Separator(sin(x) < 0.5, vars)
Separator(in(-0.5 + sin(x), [-∞, 0]), vars=x, y)

julia> S(X)
ERROR: StackOverflowError:
```