distributor: fix buffer use after free

distributor: fix buffer use after free

rte_distributor_request_pkt and rte_distributor_get_pkt dereferenced
oldpkt parameter when in RTE_DIST_ALG_SINGLE even if number
of returned buffers from worker to distributor was 0.

This patch passes NULL to the legacy API when number of returned
buffers is 0. This allows passing NULL as oldpkt parameter.

Distributor tests are also updated passing NULL as oldpkt and
0 as number of returned packets, where packets are not returned.

Fixes: 775003ad2f96 ("distributor: add new burst-capable library")
Cc: stable@dpdk.org
Signed-off-by: Lukasz Wojciechowski <l.wojciechow@partner.samsung.com>
Acked-by: David Hunt <david.hunt@intel.com>