(rewriter.flat=false) segmentation fault

z3 commit: e63dc7efc
```
$ z3release bug.smt2
unsat
$ z3release rewriter.flat=false bug.smt2 
Segmentation fault
$ cat bug.smt2
(declare-fun a () Int) 
(declare-fun b () Int) 
(declare-fun c () Int) 
(declare-fun d () Int) 
(declare-fun e () Int) 
(assert (and (>= b 0) (<= b 3) (>= c 2 d) (= c (* 2 a) d) (= (- (- e c d)) 0) (= e 1))) 
(check-sat)
```
