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

`labpc_common_detach()` is called by the comedi core to clean up if
either `labpc_attach()` (including the one in the "ni_labpc_cs" module)
or `labpc_attach_pci()` returns an error.  It assumes the `thisboard`
macro (expanding to `((struct labpc_board_struct *)dev->board_ptr)`) is
non-null.  This is a valid assumption if `labpc_attach()` fails, but not
if `labpc_attach_pci()` fails, leading to a possible NULL pointer
dereference.

Check `thisboard` at the top of `labpc_common_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>