Do not follow a NULL pointer if the tty_port_client_operations does not
implement the ->lookahead_buf() callback, which is the case with
serdev's ttyport.
Reported-by: NHans de Goede <hdegoede@redhat.com>
Fixes: 6bb6fa69 ("tty: Implement lookahead to process XON/XOFF timely")
Reviewed-by: NIlpo Järvinen <ilpo.jarvinen@linux.intel.com>
Signed-off-by: NVincent Whitchurch <vincent.whitchurch@axis.com>
Link: https://lore.kernel.org/r/20220818115026.2237893-1-vincent.whitchurch@axis.comSigned-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>