Assertion violation at ../src/util/old_vector.h Line: 370

Hi,
For this formula,
```
(assert (exists ((a Int)) (= (/ 0 0) 0)))
(check-sat-using reduce-invertible)
```
z3 throws out an assertion violation:
```
ASSERTION VIOLATION
File: ../src/util/old_vector.h
Line: 370
idx < size()
(C)ontinue, (A)bort, (S)top, (T)hrow exception, Invoke (G)DB
a
```

OS: Ubuntu 18.04
Revision: 28c827f