Only check class ancestors for ivar in memory_view

rb_class_get_superclass returns the immediate SUPER, including T_ICLASS.
rb_ivar_lookup isn't implemented for T_ICLASS so it uses the default
behaviour, which always returns Qnil.

This commit avoids checking T_ICLASS for ivars.

memory_view.c

@@ -784,7 +784,7 @@ lookup_memory_view_entry(VALUE klass)
 {
     VALUE entry_obj = rb_ivar_lookup(klass, id_memory_view, Qnil);
     while (NIL_P(entry_obj)) {
-        klass = rb_class_get_superclass(klass);
+        klass = rb_class_superclass(klass);
 
         if (klass == rb_cBasicObject || klass == rb_cObject)
             return NULL;