nat: fix coverity warning
nat: fix coverity warning

Type: fix

Change-Id: I0e87021b11009a955f5839bdb68af897145816c1
Signed-off-by: Klement Sekera <b8003f27414ca85c37624503b7b4e8f8c829bb4d@cisco.com>

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