Added the element type mjOBJ_SPEC for mjSpec.

PiperOrigin-RevId: 742678572
Change-Id: I95a382ac58b5bc88c2aa5c07210549a1d5e2f538

src/engine/engine_io.c

@@ -2143,6 +2143,7 @@ static int numObjects(const mjModel* m, mjtObj objtype) {
   case mjOBJ_DEFAULT:
   case mjOBJ_FRAME:
   case mjOBJ_UNKNOWN:
+  case mjOBJ_MODEL:
     return -1;
   case mjOBJ_BODY:
   case mjOBJ_XBODY:

src/user/user_model.cc

@@ -166,7 +166,7 @@ static void processlist(mjListKeyMap& ids, vector<T*>& list,
 // constructor
 mjCModel::mjCModel() {
   mjs_defaultSpec(&spec);
-  elemtype = mjOBJ_UNKNOWN;
+  elemtype = mjOBJ_MODEL;
   spec_comment_.clear();
   spec_modelfiledir_.clear();
   spec_meshdir_.clear();