staging: comedi: amplc_pc236: fix possible NULL deref during detach

`pc236_detach()` is called by the comedi core to clean up if either
`pc236_attach()` or `pc236_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 `pc236_attach()` fails, but not
if `pc236_attach_pci()` fails, leading to a possible NULL pointer
dereference.

Check `thisboard` at the top of `pc236_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_pc236.c

@@ -574,6 +574,8 @@ static void pc236_detach(struct comedi_device *dev)
 {
 	const struct pc236_board *thisboard = comedi_board(dev);
 
+	if (!thisboard)
+		return;
 	if (dev->iobase)
 		pc236_intr_disable(dev);
 	if (dev->irq)