defined zsuper: Handle NULL superclass for BasicObject

Prior to this commit, a segmentation fault occurred in `vm_defined`'s
`zsuper` implementation after NULL is returned as `BasicObject`'s superclass.
This fix returns false from `vm_defined` if the superclass is NULL.

For example, the following code resulted in a segfault.
```ruby
class BasicObject
  def seg_fault
    defined?(super)
  end
end

seg_fault
```

vm_insnhelper.c

@@ -5051,6 +5051,8 @@ vm_defined(rb_execution_context_t *ec, rb_control_frame_t *reg_cfp, rb_num_t op_
 
             if (me) {
                 VALUE klass = vm_search_normal_superclass(me->defined_class);
+                if (klass == (VALUE)NULL) return false;
+
                 ID id = me->def->original_id;
 
                 return rb_method_boundp(klass, id, 0);