snort: fix coverity warning
snort: fix coverity warning

Add a cast to avoid coverity warning about potential integer underflow.

Type: fix
Fixes: 839b1473e9
Signed-off-by: Klement Sekera <b8003f27414ca85c37624503b7b4e8f8c829bb4d@cisco.com>
Change-Id: Iae913e51c1e25aaeca6ce70438451c640d347383

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