NAT64: coverity fix
NAT64: coverity fix

Change-Id: I08544b2f06f993c8a6435901232a0616d0548b94
Signed-off-by: Matus Fabian <fce541bb5d92358522a3d9dac9c8f88030b6f773@cisco.com>

chrisy/vpp,chrisy/vpp,FDio/vpp,FDio/vpp,FDio/vpp,FDio/vpp,FDio/vpp,chrisy/vpp,vpp-dev/vpp,chrisy/vpp,vpp-dev/vpp,chrisy/vpp,vpp-dev/vpp,vpp-dev/vpp,chrisy/vpp,FDio/vpp,vpp-dev/vpp,vpp-dev/vpp,chrisy/vpp,chrisy/vpp,FDio/vpp,vpp-dev/vpp,FDio/vpp