netfilter: ipset: Fix cidr handling for hash:*net* types

Commit "Simplify cidr handling for hash:*net* types" broke the cidr
handling for the hash:*net* types when the sets were used by the SET
target: entries with invalid cidr values were added to the sets.
Reported by Jonathan Johnson.

Testsuite entry is added to verify the fix.

Signed-off-by: Jozsef Kadlecsik <kadlec@blackhole.kfki.hu>