qe_rec assertion violation at ../src/util/obj_hashtable.h Line: 177

Hi,
For this formula:
```
(assert (forall ((a (Array Int Bool))) (select a 0)))
(check-sat-using qe_rec)
```
Z3  throws out an assertion violation:
```
ASSERTION VIOLATION
File: ../src/util/obj_hashtable.h
Line: 177
e
(C)ontinue, (A)bort, (S)top, (T)hrow exception, Invoke (G)DB
```

OS: Ubuntu 18.04
Commit: 1aea0d2
