staging: comedi: amplc_dio200: fix possible NULL deref during detach

`dio200_detach()` is called by the comedi core to clean up if either
`dio200_attach()` or `dio200_attach_pci()` return an error.  It assigns
`thisboard` to the return value of `comedi_board(dev)` and assumes it is
non-null.  In the case of a previous call to `dio200_attach()` it won't
be `NULL` because the comedi core will have pointed it to one of the
elements of `dio200_boards[]`, but in the case of a previous call to
`dio200_attach_pci()` it could be `NULL`, leading to a null pointer
dereference.

Check that `thisboard` is valid at the top of `dio200_detach()` and
return early if it is `NULL`.  This is okay because the only other thing
that could have been allocated is `dev->private` and that is freed by
the comedi core, not by this function.

Cc: <stable@vger.kernel.org> # 3.6.x
Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/comedi/drivers/amplc_dio200.c

@@ -1410,6 +1410,8 @@ static void dio200_detach(struct comedi_device *dev)
 	const struct dio200_layout_struct *layout;
 	unsigned n;
 
+	if (!thisboard)
+		return;
 	if (dev->irq)
 		free_irq(dev->irq, dev);
 	if (dev->subdevices) {