Skip sizecheck if the size of the attribute wasn't specified, ie. zero.
The idea is providing a way to sizecheck leaf attributes and ignore the
nest (that has variable size depending on the leaves). This way we can
sizecheck at the beginning of every operation instead of polluting the
code with sizecheckings everywhere. Please, have a look at the next
patch (8/8).

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

-- 
The dawn of the fourth age of Linux firewalling is coming; a time of
great struggle and heroic deeds -- J.Kadlecsik got inspired by J.Morris