Copy default spec before assigning defaults.

PiperOrigin-RevId: 609766747
Change-Id: I79ebff62928ac2ffa5133020ee8bd001b68cdcd3

src/user/user_objects.h

@@ -1397,6 +1397,7 @@ class mjCDef {
   void Compile(const mjCModel* model);     // compiler
   mjCDef& operator=(const mjCDef& other);  // copy assignment
   void PointToLocal(void);
+  void CopyFromSpec(void);
 
   // identifiers
   std::string name;               // class name