netfilter: ipset: fix the compile warning in ip_set_create

net/netfilter/ipset/ip_set_core.c:615: warning: ‘clash’ may be used uninitialized in this function

Signed-off-by: Shan Wei <shanwei@cn.fujitsu.com>
Signed-off-by: Jozsef Kadlecsik <kadlec@blackhole.kfki.hu>
Signed-off-by: Patrick McHardy <kaber@trash.net>