examples/vhost: fix uninitialized descriptor indexes

examples/vhost: fix uninitialized descriptor indexes

Fixing the below error by returning from the function early
when count == 0.

Issue flagged by GCC 7.1.1

examples/vhost/virtio_net.c:370:38: error: ‘desc_indexes[0]’ may be used
uninitialized in this function [-Werror=maybe-uninitialized]
  rte_prefetch0(&vr->desc[desc_indexes[0]]);

Fixes: ca059fa5e290 ("examples/vhost: demonstrate the new generic APIs")
Cc: stable@dpdk.org
Signed-off-by: Jerin Jacob <jerin.jacob@caviumnetworks.com>
Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>