qfufbv_ackr invalid model on QF_BV fomula

Hi,
For this formula:
```
(declare-fun a () (_ BitVec 2))
(declare-fun b () (_ BitVec 2))
(assert (bvsle a b))
(check-sat-using qfufbv_ackr)
(get-model)
```
Z3  gives an invalid model:
```
(model 
  (define-fun b () (_ BitVec 2)
    #b10)
  (define-fun a () (_ BitVec 2)
    #b11)
)
```
if I feed this model to the formula, Z3 reports unsat.

OS: Ubuntu 18.04
Commit: b68efe4
