netfilter: nf_tables: fix potential oops when dumping sets

Commit c9c8e485 (netfilter: nf_tables: dump sets in all existing families)
changed nft_ctx_init_from_setattr() to only look up the address family if it
is not NFPROTO_UNSPEC. However if it is NFPROTO_UNSPEC and a table attribute
is given, nftables_afinfo_lookup() will dereference the NULL afi pointer.

Fix by checking for non-NULL afi and also move a check added by that commit
to the proper position.

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>