examples/vhost_scsi: fix null-check for parameter

examples/vhost_scsi: fix null-check for parameter

Coverity points out that there is a check in the main thread loop for the
ctrlr->bdev being NULL, but by that stage the pointer has already been
dereferenced. Therefore, for safety, before we enter the loop do an
initial check on the parameter structure.

Coverity issue: 158657
Fixes: db75c7af19bb ("examples/vhost_scsi: introduce a new sample app")
Cc: stable@dpdk.org
Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>
Reviewed-by: Tiwei Bie <tiwei.bie@intel.com>