Fix Java object factory recursion set

Previously it was vulnerable to indirect recursion (i.e. A has a B*,
B has an A*); this removes that possibility and also simplifies use of
the recursion set to only consider pointer edges rather than directly
nested struct types, which can't be used to construct an infinite object
graph by themselves. This should address some of the cases of prematurely
truncated object graphs the incorrect recursion test was trying to solve.

Author: smowton

src/java_bytecode/java_object_factory.cpp

@@ -316,9 +316,8 @@ void java_object_factoryt::gen_nondet_init(
     {
       const struct_typet &struct_type=to_struct_type(subtype);
       const irep_idt struct_tag=struct_type.get_tag();
-      // set to null if found in recursion set and not a sub-type
-      if(recursion_set.find(struct_tag)!=recursion_set.end() &&
-         struct_tag==class_identifier)
+      // If this is a recursive type of some kind, set null.
+      if(!recursion_set.insert(struct_tag).second)
       {
         if(update_in_place==NO_UPDATE_IN_PLACE)
           set_null(expr, pointer_type);
@@ -403,9 +402,6 @@ void java_object_factoryt::gen_nondet_init(
     if(!is_sub)
       class_identifier=struct_tag;
 
-    recursion_set.insert(struct_tag);
-    assert(!recursion_set.empty());
-
     for(const auto &component : components)
     {
       const typet &component_type=component.type();
@@ -460,7 +456,6 @@ void java_object_factoryt::gen_nondet_init(
           substruct_in_place);
       }
     }
-    recursion_set.erase(struct_tag);
   }
   else
   {