(purify-arith, ctx-solver-simplify) Soundness bug on QF_NIA formula 

Hi,
For this case, Z3  gives an incorrect answer:
```
[539] % z3release small.smt2
unsat
sat
[540] % cat small.smt2
(declare-fun c () Int)
(declare-fun a () Int)
(declare-fun b () Int)
(assert (> a 0 c (* a b)))
(assert (= b (div c a)))
(check-sat-using (then purify-arith ctx-solver-simplify))
(check-sat)
[541] %
```

OS: Ubuntu 18.04
Commit: ce07138
