Add vertex equality option.

PiperOrigin-RevId: 860299454
Change-Id: I7befd8e5f4c8bac6d74684a96a9cc3cee9ae1e29

src/engine/engine_io.c

@@ -1898,6 +1898,7 @@ const char* mj_validateReferences(const mjModel* m) {
       break;
 
     case mjEQ_FLEX:
+    case mjEQ_FLEXVERT:
       if (obj1id >= m->nflex || obj1id < 0) {
         return "Invalid model: eq_obj1id out of bounds.";
       }