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>