net/tap: fix multi process reply buffer

net/tap: fix multi process reply buffer

A successful call to rte_mp_request_sync does not guarantee that there
are any messages in the buffer, and this should be checked for before
accessing data in the message. Buffer can be empty if IPC is disabled or
if we decide to ignore replies.

Fixes: c9aa56edec8e ("net/tap: access primary process queues from secondary")
Cc: stable@dpdk.org
Signed-off-by: Herakliusz Lipiec <herakliusz.lipiec@intel.com>
Reviewed-by: Ferruh Yigit <ferruh.yigit@intel.com>