Break recursion if checking if Null is simple

[alessandropellegrini]

When checking whether a null object is Simple, a lacking check would generate infinite recursion in the method, causing a stack overflow in the symbolic executor.

I am anyhow not 100% confident that this patch is correct, so waiting for your feedback on this.

[pietrobraione]

The patch is not correct, and I am not sure the infinite recursion has anything to do with null values, but rather with circularities. Can you please provide a test case?

[pietrobraione]

Change rejected