USB: serial: mos7840: fix NULL-deref at open

commit 5c75633e 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 mos7840_open+0x88/0x8dc [mos7840]

Note that we continue to treat the interrupt-in endpoint as optional for
now.

Fixes: 3f542974 ("USB: Moschip 7840 USB-Serial Driver")
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/mos7840.c

@@ -2386,6 +2386,12 @@ static int mos7840_startup(struct usb_serial *serial)
 		return -1;
 	}
 
+	if (serial->num_bulk_in < serial->num_ports ||
+			serial->num_bulk_out < serial->num_ports) {
+		dev_err(&serial->interface->dev, "missing endpoints\n");
+		return -ENODEV;
+	}
+
 	dev = serial->dev;
 
 	dbg("%s", "Entering...");