proof Assertion violation at ../src/ast/rewriter/rewriter_def.h Line: 348

Hi,
For this formula:
```
(set-option :proof true)
(declare-datatypes () ((a (b (c Int) (d a)) f)))
(assert (forall ((e a)) (= (c e) 0)))
(check-sat)
```
Z3  throws out an assertion violation:
```
ASSERTION VIOLATION
File: ../src/ast/rewriter/rewriter_def.h
Line: 348
rewrites_to(m_r, pr1)
(C)ontinue, (A)bort, (S)top, (T)hrow exception, Invoke (G)DB
```

OS: Ubuntu 18.04
Commit: 2673807
