tty: Fix lookahead_buf crash with serdev

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: Hans de Goede <hdegoede@redhat.com>
Fixes: 6bb6fa6 ("tty: Implement lookahead to process XON/XOFF timely")
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Signed-off-by: Vincent Whitchurch <vincent.whitchurch@axis.com>
Link: https://lore.kernel.org/r/20220818115026.2237893-1-vincent.whitchurch@axis.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: vwax

drivers/tty/tty_buffer.c

@@ -470,7 +470,6 @@ static void lookahead_bufs(struct tty_port *port, struct tty_buffer *head)
 
 	while (head) {
 		struct tty_buffer *next;
-		unsigned char *p, *f = NULL;
 		unsigned int count;
 
 		/*
@@ -489,11 +488,16 @@ static void lookahead_bufs(struct tty_port *port, struct tty_buffer *head)
 			continue;
 		}
 
-		p = char_buf_ptr(head, head->lookahead);
-		if (~head->flags & TTYB_NORMAL)
-			f = flag_buf_ptr(head, head->lookahead);
+		if (port->client_ops->lookahead_buf) {
+			unsigned char *p, *f = NULL;
+
+			p = char_buf_ptr(head, head->lookahead);
+			if (~head->flags & TTYB_NORMAL)
+				f = flag_buf_ptr(head, head->lookahead);
+
+			port->client_ops->lookahead_buf(port, p, f, count);
+		}
 
-		port->client_ops->lookahead_buf(port, p, f, count);
 		head->lookahead += count;
 	}
 }