 invalid datatype declaration, segfault

Z3 segfaults on the following formula 
```
(declare-datatypes ((a 0) (b 0)))
(declare-datatypes ((c 0)) (((d (e a) (f b)))))
```

```
(error "line 1 column 33: invalid datatype declaration, '(' expected got b")
[1]    99340 segmentation fault  z3/build-2020-01-29-9694dc0/z3 bug.smt2
```
OS: Ubuntu 18.04
Revision: 9694dc0