misc: vppctl - fix coverity warning
misc: vppctl - fix coverity warning

Calculate space left to silence coverity.

Type: fix
Fixes: 31f192434660
Signed-off-by: Klement Sekera <b8003f27414ca85c37624503b7b4e8f8c829bb4d@cisco.com>
Change-Id: I9cd2e91ce74444e2625bf86721a8d3e44bf6afdd

chrisy/vpp,chrisy/vpp,chrisy/vpp,chrisy/vpp,FDio/vpp,FDio/vpp,FDio/vpp,chrisy/vpp,chrisy/vpp,FDio/vpp,chrisy/vpp,FDio/vpp,chrisy/vpp,FDio/vpp,FDio/vpp,FDio/vpp