[smart][tty] guard NULL lwp in tty_wait_background

Fix #11717

On RT-Smart a kernel thread (lwp == NULL) that reads a tty reaches
tty_wait_background() and dereferences td->lwp as a process. The shell
thread is a kernel thread while finsh reads /dev/ttyS0, so the first key
press faults on p->pgrp.

Return 0 for a NULL lwp: a kernel thread has no process group, so no
background-job decision applies. Apply the same guard to the sibling
paths that also dereference td->lwp: tty_is_ctty() returns false,
tty_drop_ctty() returns -ENOTTY, and lwp_tty_set_ctrl_proc() returns
-EPERM.

No ttydisc refactor and no change to the ioctl argument marshalling.

0 files changed, 0 insertions, 0 deletions