netfilter: ipset: fix a missing check of nla_parse

When nla_parse fails, we should not use the results (the first
argument). The fix checks if it fails, and if so, returns its error code
upstream.

Signed-off-by: Aditya Pakki <pakki001@umn.edu>
Signed-off-by: Jozsef Kadlecsik <kadlec@blackhole.kfki.hu>