Previously `ra_prof` wouldn't actually print the unaccounted time in
some cases.
We would print, for instance, this:
```
5ms - foo
2ms - bar
```
instead of:
```
5ms - foo
2ms - bar
3ms - ???
```
The fix is to properly handle the case when an entry has 0 children
instead of using the `last` variable.
Signed-off-by: Michal Terepeta <michal.terepeta@gmail.com>
e8acf49088