netfilter: nf_tables: fix possible null-pointer dereference in object update

Not all objects have an update operation. If the object type doesn't
implement an update operation and the user tries to update it will hit
EOPNOTSUPP.

Fixes: d62d0ba9

 ("netfilter: nf_tables: Introduce stateful object update operation")
Signed-off-by: Fernando Fernandez Mancera <ffmancera@riseup.net>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>