Set resume_len to 0 if download is restarted

If a download can't be resumed and is restarted, the download status messages are wrong because ```resume_len``` of the old overwritten file is added to ```data_len``` and ```byte_counter```.

E.g.:
```
$ youtube-dl ...
[download]  10.1% of 54.55MiB at 99.34KiB/s ETA 08:25
ERROR: Interrupted by user

$ youtube-dl ...
[download] Unable to resume
[download]   8.7% of 58.98MiB at 60.64KiB/s ETA 15:09
```