USB: serial: quatech2: handle unbound ports

Check for NULL port data in the modem- and line-status handlers to avoid
dereferencing a NULL pointer in the unlikely case where a port device
isn't bound to a driver (e.g. after an allocation failure on port
probe).

Note that the other (stubbed) event handlers qt2_process_xmit_empty()
and qt2_process_flush() would need similar sanity checks in case they
are ever implemented.

Fixes: f7a33e608d ("USB: serial: add quatech2 usb to serial driver")
Cc: stable <stable@vger.kernel.org>     # 3.5
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Johan Hovold <johan@kernel.org>

drivers/usb/serial/quatech2.c

@@ -841,7 +841,10 @@ static void qt2_update_msr(struct usb_serial_port *port, unsigned char *ch)
 	u8 newMSR = (u8) *ch;
 	unsigned long flags;
 
+	/* May be called from qt2_process_read_urb() for an unbound port. */
 	port_priv = usb_get_serial_port_data(port);
+	if (!port_priv)
+		return;
 
 	spin_lock_irqsave(&port_priv->lock, flags);
 	port_priv->shadowMSR = newMSR;
@@ -869,7 +872,10 @@ static void qt2_update_lsr(struct usb_serial_port *port, unsigned char *ch)
 	unsigned long flags;
 	u8 newLSR = (u8) *ch;
 
+	/* May be called from qt2_process_read_urb() for an unbound port. */
 	port_priv = usb_get_serial_port_data(port);
+	if (!port_priv)
+		return;
 
 	if (newLSR & UART_LSR_BI)
 		newLSR &= (u8) (UART_LSR_OE | UART_LSR_BI);