USB: ftdi_sio: fix null deref at port probe

Fix NULL-pointer dereference when probing an interface with no
endpoints.

These devices have two bulk endpoints per interface, but this avoids
crashing the kernel if a user forces a non-FTDI device to be probed.

Note that the iterator variable was made unsigned in order to avoid
a maybe-uninitialized compiler warning for ep_desc after the loop.

Fixes: 895f28ba ("USB: ftdi_sio: fix hi-speed device packet size
calculation")
Reported-by: Mike Remski <mremski@mutualink.net>
Tested-by: Mike Remski <mremski@mutualink.net>
Cc: <stable@vger.kernel.org>	# 2.6.31
Signed-off-by: Johan Hovold <johan@kernel.org>

drivers/usb/serial/ftdi_sio.c

@@ -1566,14 +1566,17 @@ static void ftdi_set_max_packet_size(struct usb_serial_port *port)
 	struct usb_device *udev = serial->dev;
 
 	struct usb_interface *interface = serial->interface;
-	struct usb_endpoint_descriptor *ep_desc = &interface->cur_altsetting->endpoint[1].desc;
+	struct usb_endpoint_descriptor *ep_desc;
 
 	unsigned num_endpoints;
-	int i;
+	unsigned i;
 
 	num_endpoints = interface->cur_altsetting->desc.bNumEndpoints;
 	dev_info(&udev->dev, "Number of endpoints %d\n", num_endpoints);
 
+	if (!num_endpoints)
+		return;
+
 	/* NOTE: some customers have programmed FT232R/FT245R devices
 	 * with an endpoint size of 0 - not good.  In this case, we
 	 * want to override the endpoint descriptor setting and use a