conf: Add check to avoid a NULL compare for SysfsPath

conf: Add check to avoid a NULL compare for SysfsPath

If the two sysfs_path are both NULL, there may be an incorrect
object returned for virNodeDeviceObjListFindBySysfsPath().

This check exists in old interface virNodeDeviceFindBySysfsPath().
e.g.
virNodeDeviceFindBySysfsPath(virNodeDeviceObjListPtr devs,
                             const char *sysfs_path)
{
    ...
        if ((devs->objs[i]->def->sysfs_path != NULL) &&
            (STREQ(devs->objs[i]->def->sysfs_path, sysfs_path))) {
    ...
}

Reviewed-by: Cole Robinson <crobinso@redhat.com>
Signed-off-by: Cheng Lin <cheng.lin130@zte.com.cn>