cache: check for NULL chain in cache_init()

Another process might race to add chains after chain_cache_init().
The generation check does not help since it comes after cache_init().
NLM_F_DUMP_INTR only guarantees consistency within one single netlink
dump operation, so it does not help either (cache population requires
several netlink dump commands).

Let's be safe and do not assume the chain exists in the cache when
populating the rule cache.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

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