(default) solution unsoundness on string formula with re.range

Commit: https://github.com/cvc5/cvc5/commit/6dc5b7469cee015a3bcf25a1543123da6c8317fe
OS: Ubuntu 18.04

```
[525] % z3release small.smt2
unsat
[526] % cvc5 -q small.smt2
sat
[527] % 
[527] % cat small.smt2
(assert (str.in_re "" (re.range "" "")))
(check-sat)
[528] % 
```

