fix infinite loop

for expression 
```shell
"1+2+3+4"
```
(expression ends with a digit).
if 4 is the last token, the 
```python
for (idx, ch) in remaining {
``` 
will never enter in, and offset will always be 0, this cause the `self.offset` never changed.

so when run 24_game in interactive mode, it will enter infinite loop and got no answer.

check_values will got in infinite loop, this patch fix it.
this patch seems tedious, but I haven't got better solution.