comedi: ni_atmio16d: Fix invalid clean-up after failed attach

If the driver's COMEDI "attach" handler function (`atmio16d_attach()`)
returns an error, the COMEDI core will call the driver's "detach"
handler function (`atmio16d_detach()`) to clean up.  This calls
`reset_atmio16d()` unconditionally, but depending on where the error
occurred in the attach handler, the device may not have been
sufficiently initialized to call `reset_atmio16d()`.  It uses
`dev->iobase` as the I/O port base address and `dev->private` as the
pointer to the COMEDI device's private data structure.  `dev->iobase`
may still be set to its initial value of 0, which would result in
undesired writes to low I/O port addresses.  `dev->private` may still be
`NULL`, which would result in null pointer dereferences.

Fix `atmio16d_detach()` by checking that `dev->private` is valid
(non-null) before calling `reset_atmio16d()`.  This implies that
`dev->iobase` was set correctly since that is set up before
`dev->private`.

Fixes: 2323b27 ("Staging: comedi: add ni_at_atmio16d driver")
Cc: stable <stable@kernel.org>
Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Link: https://patch.msgid.link/20260128150011.5006-1-abbotti@mev.co.uk
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: ian-abbott

drivers/comedi/drivers/ni_atmio16d.c

@@ -698,7 +698,8 @@ static int atmio16d_attach(struct comedi_device *dev,
 
 static void atmio16d_detach(struct comedi_device *dev)
 {
-	reset_atmio16d(dev);
+	if (dev->private)
+		reset_atmio16d(dev);
 	comedi_legacy_detach(dev);
 }