tty: serial_core: add tty NULL check to uart_tx_stopped

Commit 761ed4a94582 ("tty: serial_core: convert uart_close to use
tty_port_close") created a case where a port used for a console does not
get shutdown on tty closing. Then a call to uart_tx_stopped() segfaults
because the tty is NULL. This could be fixed to restore old behavior,
but we also want to allow tty_ports to work without a tty attached. So
this change to allow a NULL tty_struct is needed either way.

Fixes: 761ed4a94582 ("tty: serial_core: convert uart_close to use tty_port_close")
Reported-by: kernel test robot <[e-mail]>
Signed-off-by: Rob Herring <[e-mail]>
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

include/linux/serial_core.h

···
 static inline int uart_tx_stopped(struct uart_port *port)
 {
 	struct tty_struct *tty = port->state->port.tty;
-	if (tty->stopped || port->hw_stopped)
+	if ((tty && tty->stopped) || port->hw_stopped)
 		return 1;
 	return 0;
 }