pie: restorer -- Don't fail if no futex provided

The image data may have no futex provided but we have
no "optional" entry for that (because futex data is
a part of thread_core_entry message).

Still we know that futex length can't be zero thus
make it "optional" by a value -- if length is zero,
we consider it as a sign to skip.
Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

pie/restorer.c

@@ -205,7 +205,7 @@ static int restore_thread_common(struct rt_sigframe *sigframe,
 {
 	sys_set_tid_address((int *)decode_pointer(args->clear_tid_addr));
 
-	if (args->has_futex) {
+	if (args->has_futex && args->futex_rla_len) {
 		int ret;
 
 		ret = sys_set_robust_list(decode_pointer(args->futex_rla),