x11/konsole: avoid weird message when exiting last shell

Konsole complains about "kernel" still running in the terminal
when exiting the last shell in the last tab; this only shows
up if PID 0 is visible (depends on hardening options).
Avoid that (on Linux, apparently the invalid PID equals the
invalid foreground group ID).

PR: 258996
Reported by: alt2600@icloud.com