examples/vhost: fix use after free on drain
examples/vhost: fix use after free on drain

When a vdev is removed in destroy_device function,
the corresponding vhost TX buffer will also be freed,
but the vhost TX buffer may still be used in the
drain_vhost function, which will cause an error of
heap-use-after-free. Therefore, before accessing
vhost TX buffer, we need to check whether the vdev
has been removed, if so, let's skip this vdev.

Fixes: a68ba8e0a6b6 ("examples/vhost: refactor vhost data path")
Cc: stable@dpdk.org

Signed-off-by: Wenwu Ma <dfe818c56c28b61f539ff8934cba906f3ad4e6e7@intel.com>
Reviewed-by: Chenbo Xia <5d298a4585cd22ca5d29de25967a5599c4c324c8@intel.com>

john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk