Bugfix: Fix rare crash in command output

Some commands could in rare cases crash when trying to print status messages
and request retries at the same time, resulting in an error like the following:

```
panic: runtime error: slice bounds out of range [468:156]
[...]
github.com/restic/restic/internal/ui/termstatus.(*lineWriter).Write(...)
	/restic/internal/ui/termstatus/stdio_wrapper.go:36 +0x136
```

This has now been fixed.

https://github.com/restic/restic/issues/5259
https://github.com/restic/restic/pull/5300