um: Remove redundant NULL check

Fix below warnings reported by coccicheck:
./arch/um/drivers/vector_user.c:403:2-7: WARNING: NULL check before some freeing functions is not needed.

Fixes: bc8f8e4e ("um: Add a generic "fd" vector transport")
Signed-off-by: Li Heng <liheng40@huawei.com>
Signed-off-by: Richard Weinberger <richard@nod.at>

arch/um/drivers/vector_user.c

@@ -397,8 +397,7 @@ static struct vector_fds *user_init_fd_fds(struct arglist *ifspec)
 fd_cleanup:
 	if (fd >= 0)
 		os_close_file(fd);
-	if (result != NULL)
-		kfree(result);
+	kfree(result);
 	return NULL;
 }