process-util: a process from a foreign pidns is definitely not our child

Addresses: https://github.com/systemd/systemd/pull/35242#pullrequestreview-2531712318

src/basic/process-util.c

@@ -1096,6 +1096,8 @@ int pidref_is_my_child(const PidRef *pid) {
 
         pid_t ppid;
         r = pidref_get_ppid(pid, &ppid);
+        if (r == -EADDRNOTAVAIL) /* if this process is outside of our pidns, it is definitely not our child */
+                return false;
         if (r < 0)
                 return r;