usb_serial: Fix oops when unexisting usb serial device is opened.

This commit 335f8514 has stopped
properly checking if there is any usb serial associated with the tty in
the close function. It happens the close function is called by releasing
the terminal right after opening the device fails.

As an example, open fails with a non-existing device, when probe has
never been called, because the device has never been plugged. This is
common in systems with static modules and no udev.
Signed-off-by: NThadeu Lima de Souza Cascardo <cascardo@holoscopio.com>
Signed-off-by: NAlan Cox <alan@linux.intel.com>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

drivers/usb/serial/usb-serial.c

@@ -333,6 +333,9 @@ static void serial_close(struct tty_struct *tty, struct file *filp)
 {
 	struct usb_serial_port *port = tty->driver_data;
 
+	if (!port)
+		return;
+
 	dbg("%s - port %d", __func__, port->number);