horn assertion violation at ../src/muz/base/dl_context.cpp Line: 691

Hi,
For this formula:
```
(declare-fun a () Bool)
(assert (or a (and a)))
(check-sat-using horn)

```
Z3  throws out an assertion violation:
```
ASSERTION VIOLATION
File: ../src/muz/base/dl_context.cpp
Line: 691
m_closed
(C)ontinue, (A)bort, (S)top, (T)hrow exception, Invoke (G)DB
```

OS: Ubuntu 18.04
Commit: 85fd139
