z3str3 assertion violation at ../src/ast/ast.cpp Line: 431

Hi,
For this formula,
```
(declare-const a String)
(assert (< (str.to.int a) 1))
(check-sat)
```
`z3 smt.string_solver=z3str3` throws out an assertion violation:
```
ASSERTION VIOLATION
File: ../src/ast/ast.cpp
Line: 431
UNREACHABLE CODE WAS REACHED.
(C)ontinue, (A)bort, (S)top, (T)hrow exception, Invoke (G)DB
```

OS: Ubuntu 18.04
Revision: 140926e