USB: cdc-acm: fix potential null-pointer dereference on disconnect

Fix potential null-pointer exception on disconnect introduced by commit
11ea859d64b69a747d6b060b9ed1520eab1161fe (USB: additional power savings
for cdc-acm devices that support remote wakeup).

Only access acm->dev after making sure it is non-null in control urb
completion handler.

Cc: stable <stable@kernel.org>
Signed-off-by: Johan Hovold <jhovold@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

drivers/usb/class/cdc-acm.c

@@ -297,6 +297,8 @@ static void acm_ctrl_irq(struct urb *urb)
 	if (!ACM_READY(acm))
 		goto exit;
 
+	usb_mark_last_busy(acm->dev);
+
 	data = (unsigned char *)(dr + 1);
 	switch (dr->bNotificationType) {
 	case USB_CDC_NOTIFY_NETWORK_CONNECTION:
@@ -336,7 +338,6 @@ static void acm_ctrl_irq(struct urb *urb)
 		break;
 	}
 exit:
-	usb_mark_last_busy(acm->dev);
 	retval = usb_submit_urb(urb, GFP_ATOMIC);
 	if (retval)
 		dev_err(&urb->dev->dev, "%s - usb_submit_urb failed with "