Add signature error checking at the end of compilation.

Fixed some edge cases that caused the new error to be triggered.

PiperOrigin-RevId: 740483759
Change-Id: I6ced3ba55476d5c18103e205151ecace94f2b29b

src/user/user_objects.cc

@@ -1422,6 +1422,7 @@ mjCFrame* mjCBody::ToFrame() {
       parent->bodies.end());
   model->ResetTreeLists();
   model->MakeTreeLists();
+  model->spec.element->signature = model->Signature();
   return newframe;
 }