qe2 z3str3 assertion violation at ../src/qe/qsat.cpp Line: 633

Hi,
For this formula:
```
(declare-fun s () String)
(assert (< (str.to.int s) 2))
(check-sat-using (then qe2 smt))
```
Z3  throws out an assertion violation:
```
ASSERTION VIOLATION
File: ../src/qe/qsat.cpp
Line: 633
validate_defs("check_sat")
(C)ontinue, (A)bort, (S)top, (T)hrow exception, Invoke (G)DB
```

OS: Ubuntu 18.04
Commit: 3bb05b5
