netfilter: nft_fib_ipv4: initialize *dest to zero

Otherwise, if fib lookup fail, *dest will be filled with garbage value,
so reverse path filtering will not work properly:
 # nft add rule x prerouting fib saddr oif eq 0 drop

Fixes: f6d0cbcf ("netfilter: nf_tables: add fib expression")
Signed-off-by: Liping Zhang <zlpnobody@gmail.com>
Acked-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>