Completeness regression: Missed rewrite of `(str.substr _ 0 0)`?

Commit: dc6a8fde34d62492853a28979b287cc15c62bd8c

```
$ z3-4.8.10 unknown.smt2
unsat
$ z3 unknown.smt2
unknown
$ cat unknown.smt2
(declare-fun c () Int)
(assert (forall ((v Int)) (and (= v 0) (not (str.contains (str.substr "ZJVF" 0 0) (str.substr "ZJVF" 0 c))))))
(check-sat)
```