smt segmentation fault on QF_BV formula with bvslt

Hi,
For this formula:
```
(declare-fun a () (_ BitVec 9000))
(declare-fun b () (_ BitVec 9000))
(assert (bvslt a b))
(check-sat-using smt)

```
Z3  throws out a segmentation fault.

OS: Ubuntu 18.04
Commit: dddd740
