Fix #143600: Library overrides on custom properties are broken.

Mistake in 7276b2009a, where iterator over StructRNA properties was
changed to iterate over the new system IDProps, instead of the existing
(now 'custom data'-only) user IDProps.

System IDProps do not need to be handled in this iterator, as they are
expected to already be processed through their wrapping RNA properties.

source/blender/makesrna/intern/rna_rna.cc

@@ -458,9 +458,13 @@ static void rna_Struct_properties_next(CollectionPropertyIterator *iter)
     /* regular properties */
     rna_inheritance_properties_listbase_next(iter, rna_property_builtin);
 
-    /* try id properties */
+    /* Try IDProperties (i.e. custom data).
+     *
+     * NOTE: System IDProperties should not need to be handled here, as they are expected to have a
+     * valid (runtime-defined) RNA property to wrap them, which will have been processed above as
+     * part of `rna_inheritance_properties_listbase_next`. */
     if (!iter->valid) {
-      group = RNA_struct_system_idprops(&iter->builtin_parent, 0);
+      group = RNA_struct_idprops(&iter->builtin_parent, 0);
 
       if (group) {
         rna_iterator_listbase_end(iter);