netfilter: nf_conntrack: fix error return code

Fix to return a negative error code from the error handling
case instead of 0, as returned elsewhere in function
nf_conntrack_standalone_init().

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>