hash: check if slot is empty with key index

hash: check if slot is empty with key index

Instead of checking if the current and alternative signatures are 0,
it is faster to check if the key index associated to an entry
is 0, meaning that the slot is empty.

Signed-off-by: Pablo de Lara <pablo.de.lara.guarch@intel.com>
Acked-by: Saikrishna Edupuganti <saikrishna.edupuganti@intel.com>