bug(sum): when the filename is /dev/drm_dp_aux2, it skips all arguments

Hi, uutils mainteners

we confirmed a serious bug when using the filename `/dev/drm_dp_aux2`, as my test confirmed

```css
relunsec@relunsec:~/software/coreutils/target/debug$ sudo ./sum /dev/drm_dp_aux2 a
sum: Timed out
relunsec@relunsec:~/software/coreutils/target/debug$ sudo gnusum /dev/drm_dp_aux2 a
gnusum: /dev/drm_dp_aux2: Connection timed out
49170     1 a
```