Add error checking in mjs_setFrame.

Also raise error if the frame is not found in the mjCBody copy constructor.

Fixes #2543.

PiperOrigin-RevId: 742624013
Change-Id: I4999b3165c97f8d079412214d027ca7a8dae8cb3

src/user/user_objects.h

@@ -254,6 +254,9 @@ class mjCBase : public mjCBase_ {
   // Copy plugins instantiated in this object
   virtual void CopyPlugin() {}
 
+  // Returns parent of this object
+  virtual mjCBase* GetParent() const { return nullptr; }
+
   // Copy assignment
   mjCBase& operator=(const mjCBase& other);