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: NLiping Zhang <zlpnobody@gmail.com>
Acked-by: NFlorian Westphal <fw@strlen.de>
Signed-off-by: NPablo Neira Ayuso <pablo@netfilter.org>

net/ipv4/netfilter/nft_fib_ipv4.c

@@ -122,6 +122,8 @@ void nft_fib4_eval(const struct nft_expr *expr, struct nft_regs *regs,
 		fl4.saddr = get_saddr(iph->daddr);
 	}
 
+	*dest = 0;
+
 	if (fib_lookup(nft_net(pkt), &fl4, &res, FIB_LOOKUP_IGNORE_LINKSTATE))
 		return;