avoid illegal memory access in destructor after variables have been deleted

src/variable.cpp

@@ -1213,6 +1213,9 @@ void Variable::remove(int n)
     dvalue[i-1] = dvalue[i];
   }
   nvar--;
+  data[nvar] = nullptr;
+  reader[nvar] = nullptr;
+  names[nvar] = nullptr;
 }
 
 /* ----------------------------------------------------------------------