sony-laptop: fix early NULL pointer dereference

The SNC acpi driver could get early notifications before it fully
initializes and that could lead to dereferencing the sony_nc_handles
structure pointer that is still NULL at that stage.
Make sure we return early from the handle lookup function in these
cases.

Signed-off-by: Mattia Dongili <malattia@linux.it>
Signed-off-by: Matthew Garrett <mjg@redhat.com>