[PATCH] tty: Fix lookahead_buf crash with serdev

[Vincent Whitchurch]

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.

Fixes: 6bb6fa6908ebd3 ("tty: Implement lookahead to process XON/XOFF timely")
Signed-off-by: Vincent Whitchurch <vincent.whitchurch@xxxxxxxx>
---
 drivers/tty/tty_buffer.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/tty/tty_buffer.c b/drivers/tty/tty_buffer.c
index 9fdecc795b6b..a1c97d4a45fb 100644
--- a/drivers/tty/tty_buffer.c
+++ b/drivers/tty/tty_buffer.c
@@ -493,7 +493,8 @@ static void lookahead_bufs(struct tty_port *port, struct tty_buffer *head)
 		if (~head->flags & TTYB_NORMAL)
 			f = flag_buf_ptr(head, head->lookahead);
 
-		port->client_ops->lookahead_buf(port, p, f, count);
+		if (port->client_ops->lookahead_buf)
+			port->client_ops->lookahead_buf(port, p, f, count);
 		head->lookahead += count;
 	}
 }
-- 
2.34.1