USB: serial: oti6858: fix NULL-deref at open

commit 5afeef23 upstream.

Fix NULL-pointer dereference in open() should the device lack the
expected endpoints:

Unable to handle kernel NULL pointer dereference at virtual address 00000030
...
PC is at oti6858_open+0x30/0x1d0 [oti6858]

Note that a missing interrupt-in endpoint would have caused open() to
fail.

Fixes: 49cdee0e ("USB: oti6858 usb-serial driver (in Nokia CA-42
cable)")
Signed-off-by: Johan Hovold <johan@kernel.org>
[bwh: Backported to 3.2: add this check to the existing
 usb_serial_driver::attach implementation]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

drivers/usb/serial/oti6858.c

@@ -347,9 +347,17 @@ static void send_data(struct work_struct *work)
 static int oti6858_startup(struct usb_serial *serial)
 {
 	struct usb_serial_port *port = serial->port[0];
+	unsigned char num_ports = serial->num_ports;
 	struct oti6858_private *priv;
 	int i;
 
+	if (serial->num_bulk_in < num_ports ||
+			serial->num_bulk_out < num_ports ||
+			serial->num_interrupt_in < num_ports) {
+		dev_err(&serial->interface->dev, "missing endpoints\n");
+		return -ENODEV;
+	}
+
 	for (i = 0; i < serial->num_ports; ++i) {
 		priv = kzalloc(sizeof(struct oti6858_private), GFP_KERNEL);
 		if (!priv)