staging: comedi: das08: fix possible NULL deref during detach

`das08_detach()` is called by the comedi core to clean up if either
`das08_attach()` or `das08_attach_pci()` returns an error.  It sets
`thisboard` to the return value of `comedi_board(dev)` and assumes it is
non-null.  This is a valid assumption if `das08_attach()` fails, but not
if `das08_attach_pci()` fails, leading to a possible NULL pointer
dereference.

Check `thisboard` at the top of `das08_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/das08.c

@@ -846,6 +846,8 @@ static void __maybe_unused das08_detach(struct comedi_device *dev)
 {
 	const struct das08_board_struct *thisboard = comedi_board(dev);
 
+	if (!thisboard)
+		return;
 	das08_common_detach(dev);
 	if (is_isa_board(thisboard)) {
 		if (dev->iobase)