staging: comedi: ni_tio: make ni_gpct_device_destroy() NULL pointer safe

Modify the pointer check to make this function NULL pointer safe.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>