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: NCole Robinson <crobinso@redhat.com>
Signed-off-by: NCheng Lin <cheng.lin130@zte.com.cn>

src/conf/virnodedeviceobj.c

@@ -207,7 +207,8 @@ virNodeDeviceObjListFindBySysfsPathCallback(const void *payload,
     int want = 0;
 
     virObjectLock(obj);
-    if (STREQ_NULLABLE(obj->def->sysfs_path, sysfs_path))
+    if (obj->def->sysfs_path &&
+        STREQ_NULLABLE(obj->def->sysfs_path, sysfs_path))
         want = 1;
     virObjectUnlock(obj);
     return want;