vhost: fix null pointer checking

vhost: fix null pointer checking

Null value for parameters will cause segfault.

Fixes: d7280c9fffcb ("vhost: support selective datapath")
Fixes: 72e8543093df ("vhost: add API to get MTU value")
Fixes: a277c7159876 ("vhost: refactor code structure")
Fixes: ca33faf9ef10 ("vhost: introduce API to fetch negotiated features")
Fixes: eb32247457fe ("vhost: export guest memory regions")
Fixes: 40ef286f236a ("vhost: export vhost vring info")
Fixes: bd2e0c3fe5ac ("vhost: add APIs for live migration")
Fixes: 0b8572a0c101 ("vhost: add external message handling to the API")
Fixes: b4953225cea4 ("vhost: add APIs for datapath configuration")
Fixes: 5fbb3941da9f ("vhost: introduce driver features related APIs")
Fixes: 292959c71961 ("vhost: cleanup unix socket")
Cc: stable@dpdk.org
Signed-off-by: Mohammad Abdul Awal <mohammad.abdul.awal@intel.com>
Reviewed-by: Tiwei Bie <tiwei.bie@intel.com>