Goto analyzer: do not create untyped nil expression

We need to create a typed variant of top rather than relying on eval to
produce a typed expression. Tests involving pointer difference
spuriously passed for 64-bit platforms include a type cast in those
expressions. The abstract evaluation of said type cast resulted in typed
expressions despite the untyped nil input.

Author: tautschnig

src/analyses/variable-sensitivity/abstract_pointer_object.cpp

@@ -99,7 +99,7 @@ abstract_object_pointert abstract_pointer_objectt::eval_ptr_diff(
   const namespacet &ns) const
 {
   if(is_top() || operands[1]->is_top())
-    return environment.eval(nil_exprt(), ns);
+    return environment.abstract_object_factory(expr.type(), ns, true, false);
 
   return ptr_diff(expr, operands, environment, ns);
 }