netfilter: ipset: fix hash:net,port,net hang with /0 subnet

The hash:net,port,net set type supports /0 subnets. However, the patch
commit 5f7b51bf titled "netfilter: ipset: Limit the maximal range
of consecutive elements to add/delete" did not take into account it and
resulted in an endless loop. The bug is actually older but the patch
5f7b51bf brings it out earlier.

Handle /0 subnets properly in hash:net,port,net set types.

Fixes: 5f7b51bf ("netfilter: ipset: Limit the maximal range of consecutive elements to add/delete")
Reported-by: Марк Коренберг <socketpair@gmail.com>
Signed-off-by: Jozsef Kadlecsik <kadlec@netfilter.org>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>