hash: fix incorrect lookup if key is all zero

hash: fix incorrect lookup if key is all zero

If user has not added an all zero key in the hash table,
and tries to look it up, it results in an incorrect hit,
as dummy slot in the key table has all zero as well.

Fixes: 48a399119619 ("hash: replace with cuckoo hash implementation")

Signed-off-by: Pablo de Lara <pablo.de.lara.guarch@intel.com>
Acked-by: Bruce Richardson <bruce.richardson@intel.com>