sfds: Fix UB in choose_service_fd_base due to calling __builtin_clz(0)

__builtin_clz(0) leads to undefined behaviour:
https://gcc.gnu.org/onlinedocs/gcc/Other-Builtins.html

Set nr = 1 directly to avoid this.

Link: https://github.com/checkpoint-restore/criu/issues/470Signed-off-by: Radoslaw Burny <rburny@google.com>
Acked-by: Kirill Tkhai <ktkhai@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>

criu/util.c

@@ -591,7 +591,10 @@ static int choose_service_fd_base(struct pstree_item *me)
 	real_nr = nr;
 
 	nr /= (1024 / sizeof(void *));
+	if (nr)
 		nr = 1 << (32 - __builtin_clz(nr));
+	else
+		nr = 1;
 	nr *= (1024 / sizeof(void *));
 
 	if (nr > service_fd_rlim_cur) {