dfmc-modeling: Improve access to meta-object slots

* sources/dfmc/modeling/objects.dylan
  (<implementation-class>): Add a default initialization value to the
   direct-superclasses slot so that the compiler will consider it as
   guaranteed-initialized.

Author: housel

sources/dfmc/modeling/objects.dylan

@@ -456,6 +456,7 @@ define concrete primary &class <implementation-class> (<object>)
     init-value: default-class-constructor;
 
   lazy &slot direct-superclasses :: <simple-object-vector>,
+    init-value: #[],
     init-keyword: superclasses:;
 
   // RCPL fast subclass slots, markt, 2-Apr-97