staging: comedi: amplc_pc263: fix possible NULL deref during detach

`pc263_detach()` is called by the comedi core to clean up if either
`pc263_attach()` or `pc263_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 `pc263_attach()` fails, but not
if `pc263_attach_pci()` fails, leading to a possible NULL pointer
dereference.

Check `thisboard` at the top of `pc263_detach()` and return early if it
is `NULL`.  This is okay because no other resources need cleaning up in
this case.

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>