can't play on completed boards

uttt.c

@@ -89,6 +89,9 @@ int ut_move(struct ut_state *new_state, const struct ut_state *old_state, int ro
 	// bad move - tile is occupied
 	if(old_state->tiles[row][col] != '\0') {return 1;} // replace with ' ' maybe
 
+	// bad move - board is not playable
+	if(old_state->boards[row / 3][col / 3] != '\0') {return 1;} // replace with ' ' maybe
+
 	// copy old_state->{tiles,boards} to new_state->{tiles,boards}
 	memmove(new_state->tiles, old_state->tiles, sizeof(old_state->tiles));
 	memmove(new_state->boards, old_state->boards, sizeof(old_state->boards));