software node: Initialize the return value in software_node_to_swnode()

The software node is searched from a list that may be empty
when the function is called. This makes sure that the
function returns NULL even if the list is empty.

Fixes: 80488a6b ("software node: Add support for static node descriptors")
Reported-by: kbuild test robot <lkp@intel.com>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>