proof assertion violation at ../src/ast/ast.cpp Line: 2828

Hi,
For this formula:
```
(set-option :proof true) 
(declare-fun a () Int)     
(declare-fun b () Int) 
(assert (>= a 0 a ( * a b))) 
(check-sat) 
```
Z3  throws out an assertion violation:
```
ASSERTION VIOLATION
File: ../src/ast/ast.cpp
Line: 2828
to_app(get_fact(p2))->get_arg(0) == get_fact(p1)
(C)ontinue, (A)bort, (S)top, (T)hrow exception, Invoke (G)DB
```

OS: Ubuntu 18.04
Commit: 6f2b569
