Refutation soundness bug in string logic 

z3 commit: e63dc7efc

```
$ cvc4 -q --strings-exp bug.smt2
sat
$ z3-4.8.10 bug.smt2
sat
$ z3release bug.smt2
unsat
$ cat bug.smt2
(declare-fun a() String)
(declare-fun b() String)
(assert (> (ite (str.prefixof "-" (str.at b 0)) 0 (str.to_int (str.at b (- (str.len b) (str.len a))))) 0))
(check-sat)
```
