a potential IndexError

In line 124, since you write
```python
while True:
    ...
    if front == rcvd_ack.ack_num:
        break;
```
The `if` statement means the receiver haven't receive the packet with first byte number of ack_num.
However, if the ack_num is not in the window since it always ack the next byte, maybe it will raise an Index Error