distributor: fix return pkt calls in single mode

distributor: fix return pkt calls in single mode

In the single legacy version of the distributor synchronization
requires continues exchange of buffers between distributor
and workers. Empty buffers are sent if only handshake
synchronization is required.
However calls to the rte_distributor_return_pkt()
with 0 buffers in single mode were ignored and not passed to the
legacy algorithm implementation causing lack of synchronization.

This patch fixes this issue by passing NULL as buffer which is
a valid way of sending just synchronization handshakes
in single mode.

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>