Debug branch: qe-sat Assertion violation at ../src/ast/ast.h Line: 721

Hi,
For this formula:
```
(declare-fun a (Int Int) Bool) 
(assert (forall ((b Int)) 
(xor (a 1 b)))) 
(check-sat-using qe-sat) 
```
Z3 debug branch  throws out an assertion violation:
```
ASSERTION VIOLATION
File: ../src/ast/ast.h
Line: 721
idx < m_num_args
(C)ontinue, (A)bort, (S)top, (T)hrow exception, Invoke (G)DB
```

OS: Ubuntu 18.04
Commit: d4b137a
