USB: keyspan: Fix oops

If we get a data URB back from the hardware after we have put the tty to
bed we go kaboom. Fortunately all we need to do is process the URB
without trying to ram its contents down the throat of an ex-tty.

Cc: stable <stable@kernel.org>
Signed-off-by: NAlan Cox <alan@redhat.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@suse.de>

drivers/usb/serial/keyspan.c

@@ -838,7 +838,7 @@ static void	usa49_indat_callback(struct urb *urb)
 
 	port = (struct usb_serial_port *) urb->context;
 	tty = port->tty;
-	if (urb->actual_length) {
+	if (tty && urb->actual_length) {
 		/* 0x80 bit is error flag */
 		if ((data[0] & 0x80) == 0) {
 			/* no error on any byte */