netfilter: nft_osf: check if attribute is present

If the attribute is not sent, eg. old libnftnl binary, then
tb[NFTA_OSF_TTL] is NULL and kernel crashes from the _init path.

Fixes: a218dc82 ("netfilter: nft_osf: Add ttl option support")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>