tty: clear cdev pointer after cdev_add() failure

tty_cdev_add() drops the cdev reference when cdev_add() fails, but
leaves driver->cdevs[index] pointing to freed memory.
tty_unregister_device() later passes that stale pointer to cdev_del(),
causing a use-after-free.

Clear the slot after dropping the reference.

Fixes: c1a752ba2d6b ("tty: don't leak cdev in tty_cdev_add()")
Assisted-by: Claude:claude-fable-5
Signed-off-by: Karl Mehltretter <kmehltretter@gmail.com>
Link: https://patch.msgid.link/20260731181844.11330-5-kmehltretter@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>