handle negative values

src/smq/snmp/snmp.go

@@ -122,6 +122,8 @@ func Start() {
                 if err != nil {
                   log.Printf("Error when building difference: %v", err)
                   convertedValue = "-1"
+                } else if diff < 0 {
+                  convertedValue = "-2"
                 } else {
                   convertedValue = diff 
                 }