Debug branch: qe assertion violation at ../src/qe/qe.cpp Line: 1764 with 2 threads

Hi,
For this formula:
```
(set-option :smt.threads 2)
(assert (forall ((a Int) (b Int)) 
(exists ((x Int)) (and (< a (* 20 x)) (< (* 20 x) b))))) 
(apply qe)


```
Z3 debug branch  throws out an assertion violation:
```
ASSERTION VIOLATION
File: ../src/qe/qe.cpp
Line: 1764
l_false != m_solver.check()
(C)ontinue, (A)bort, (S)top, (T)hrow exception, Invoke (G)DB
```

OS: Ubuntu 18.04
Commit: 9b58b77
