SNAT: fix coverity warnings (VPP-608)
SNAT: fix coverity warnings (VPP-608)

Change-Id: Id7879a6a8014fe57c3515a13e7597fb0e3c906ad
Signed-off-by: Matus Fabian <fce541bb5d92358522a3d9dac9c8f88030b6f773@cisco.com>

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