Fix SSCC-18 checksum
Fix SSCC-18 checksum

bwipp/postscriptbarcode,bwipp/postscriptbarcode,bwipp/postscriptbarcode,bwipp/postscriptbarcode,bwipp/postscriptbarcode,bwipp/postscriptbarcode,bwipp/postscriptbarcode,bwipp/postscriptbarcode