Fixed entailment of PropBoolChannel when elements in the set is negative...

... or larger than the length of the boolean array.

For example, the following code will throw an array out of index exception.

```java
public static void main(String[] args) {
    Solver solver = new Solver();
    BoolVar[] bs = VF.boolArray("bs", 3, solver);
    SetVar s1 = VF.set("s1", -3, 3, solver);
    SetVar s2 = VF.set("s2", -3, 3, solver);
    solver.post(LCF.or(SCF.all_equal(new SetVar[]{s1, s2}), SCF.bool_channel(bs, s1, 0)));
    if (solver.findSolution()) {
        do {
            System.out.println(s1 + " : " + s2);
        } while (solver.nextSolution());
    }
}
```