(qe) Soundness bug on (assert (forall ((a Int)) (distinct (mod a a) a)))

Hi,
For this case, Z3  gives an incorrect answer:
```
[528] % z3 small.smt2 
sat
unsat
[529] % cat small.smt2 
(assert (forall ((a Int)) (distinct (mod a a) a)))
(check-sat)
(check-sat-using qe)
[530] %
```

OS: Ubuntu 18.04
Commit: 7f1b147
